OK, before I begin. Redone; it's obvious that you're concerned for your friend and that is definitely a good thing. You seem to be worried about her welfare and you appear scared of what might happen to her if she meets this guy. I respect you for that. I understand why you're trying to work out what to do. Please try to remember that, since some of the following might otherwise come across as a little harsher than it's intended to be.
(Also, in spite of the probable length of this, I'm quite lazy so rather than type ex-model, I'm sticking to "model" and the online boyfriend/dom is hereafter "the guy". The husband still gets to be "the husband").
To begin with, there's nothing in your post that makes it look like the woman concerned has to be told anything. Her relationship with the guy pre-dates your involvement. If she's asking permission for all of the things you mentioned, you have to assume that she has made the choice to allow this guy to dominate her. You might not understand this choice, you might not like it but I hope you will acknowledge that it is her choice and her choice alone. If she didn't want him to control her, she wouldn't let him.
Whether her husband is aware of the situation is also, to be blunt, absolutely none of your business. For the sake of everyone involved, I hope her husband does know but it's none of your concern. That is a matter which is strictly between her and her husband (it's not even the guy's business, it's certainly not yours). Lots of people in this world make choices and decisions that will never make sense to other people but it is still their right to make those choices. You have to respect that, even when you feel, think or even "know" that their choices are "wrong". If you cannot contain yourself and feel you have to warn her about things she already seems to be fully aware of, you will have to accept that you will in all likelihood lose whatever friendship you have with her.
I completely understand that her situation doesn't make sense to you. Frankly, it doesn't make sense to me either. There's very, very little of the BDSM world that I am close to understanding or have any desire to ever participate in but that doesn't necessarily mean it is actually wrong. It just means that there are people out there in the big wide world who have lived lives and made choices which make no sense to me. It doesn't mean that they have made bad choices or are unaware of what they are doing, it simply means that I don't relate to their point of view. To think otherwise is to presume that you know better than they do about their own freaking lives. Besides, even if people make bad decisions, surely they can face the consequences for themselves, try to learn from their mistakes and get on with their own lives?
If you asked my opinion of the guy, I would tell you that he seems to be a bit of an asshole. But I don't actually know him, I don't know why he acts the way he does and so I can't even say that for certain. Neither can you. I suspect I wouldn't like him but for all I know, the guy spends his spare time performing free cataract surgery on puppies. Outside of his controlling behaviour with her, he might be the nicest guy on the planet. People are complicated, with many facets to their personalities. As tempting as it is, you can't judge him by this behaviour alone. Unless someone is harming you, your friends or your family against their will then you shouldn't judge them at all really. Even then, you probably shouldn't judge them. (Although in those situations, I have to admit that I sometimes do anyway).
I actually have a friend who is quite heavily into the whole BDSM scene. Before I knew him he helped start a dungeon for like-minded types to roleplay and do what they enjoy. He has spent the better part of twenty years in a relationship with a woman who has done things like refuse to allow him to brush his teeth or go to a dentist even though he lived in constant pain from cavities and tooth decay for years.
Do I understand it? Fuck no.
Did I try to help? Yep. I told him that I thought he was in a destructive relationship which would never do him any good.(I was much more self-righteous and judgmental when I was younger, but I got better.)
Did what I said matter? Nope, not a jot. He got defensive and so I backed off. Ultimately, it came down to the fact that he was doing what he wanted to do for reasons that I couldn't understand but which made sense to him and I realised that I had to let him live his life and wait until he asked for help, if he wanted it.
Thankfully, we did manage to remain friends (although it was a bit rocky for a while, even though at the time I thought that all I thought I had done was to express concern for his welfare). Like Rosemary felt compelled to speak up in her post above (and for far,far less reason than she had), I spoke my mind, made my point and left them to it. Ten years on, he's still with his girlfriend in a relationship that makes utterly no sense to me but it works for him and that's the main thing.
Ultimately, in my opinion one of the marks of being a true friend to someone is supporting them even when their choices make no sense to you. It's part of being a friend. Obviously there are exceptions to this (the biggest I can think of is child neglect or abuse) but on the whole, you have to let people live their own lives even when their choices make no sense to you.
As a personal aside, out of the hundreds of relationships I've seen around me over my whole life, hardly any of them have ever really make much sense to me. The overwhelming majority of friends and acquaintances I have known have stayed in situations that I would have run a mile from, where they appear(ed) to be compromising things like happiness or freedom in ways that I have almost never understood and hopefully never will.
However, short of genuinely abusive scenarios where I fear for their safety, it's none of my business and even then, I have had to learn to accept that it's none of my business. As long as I let them know that I'm there for them if they need help (and actually follow through on that should the need arise), then their lives are their lives. If they ask me for an opinion about their situation, I will almost certainly be honest with them, but until they ask it's really best not to tell them, however tempting or useful you think it might be.
The last thing I want to address is your concern over what might happen if or when they meet. I understand that, from your point of view, the difference between this level of domination in a relationship and actual physical abuse is possibly non-existent. But, from my admittedly limited personal experience or reading on the subject, there is no reason to expect that she will have any greater threat of physical harm if they do happen to meet than any other two netfriends do.
(While I was writing this, lordmagellan posted just above me, he covered this point much better than I did, so what he said, can't be stuffed editing now lol).
When it comes down to it, you just have to hope that she knows what she's doing and let her live her own life the way she chooses. That can be tough, but you have to accept her right to make her own choices.
So, short version; it's none of your business. If you have to say anything, just let her know you're there as a friend if she ever needs you, but otherwise..... Stay the fuck out of it and get on with making your own choices and possibly even your own mistakes.
And a special ps to Jupiter who also posted while I wrote this; as you say, you're coming to this as an outsider. So am I. However, if it's working for the people involved, it's not necessarily dysfunctional. What caused the desire for this type of thing is beyond our ken, you and I don't know what it's based on. Just because it doesn't work for you or me, you cannot say if it works for others, sorry mate.