To me, it depends on why she changed her mind. If they agreed to have vaginal sex, and then he tries to stick it in her ass and keeps insisting (for example), I don't think he should get his money back. I think he forfeited that right when he went back on their verbal agreement. Otherwise if she stops I think she should refund him. Either way it's rape. It becomes kind of a sticky situation if all the details weren't discussed enough, though. It's hard to cover all your bases.
Proving any of this is court... would be hard, though. Verbal contracts don't mean much, and no one's going to keep clear paperwork of their illegal business. If prostitution were more accepted and legal, I think a pretty fair way to do this would be to keep audio recordings of sessions, for the financial and physical safety of both involved.
ACFFAN, I don't think he should get a refund in that case. If they agreed "we have 30 minutes together, and in that time we can have sex" and he starts having sex with her 28 minutes in and doesn't finish in those 2 minutes that are left, that's his fault. You're paying for her time as much as for the sex. Now, a conscientious escort would push for him to have sex earlier than later if that's what she thought he wanted, but if he keeps dragging his feet she shouldn't be expected to be responsible for that, in my opinion.
Edit: You edited your post after I wrote that up, I agree that only the second half should be refunded then, so long as he didn't do anything wrong within that period.
