Horses for courses - I would never have accepted 3K unless I knew I had earned it.
I would never have accepted the whole of the 3k either if i know i am not good with money and a chance i will spend it.
There is a lot of people out there not just sex workers all walks of life that would find it hard not to dip into it that is why best not to be tempted in first place.
The problem she has is him knowing where she lives so she cannot run off anyway. I think if he cannot or will not accept installments then not much she can do. If he continues to make threats she might need to call the local police explain.the situation that she made a bad call but trying to put it right and he will not except that and is making her scared. Police will say he has a right to ask for money back but not make threats or scare you. They could tell him he needs to go small claims and let a judge decide if she needs to pay anything back depending what proof there is of what the money was for.
She said he is wealthy so he could afford to accept installments but sounds like it is more about anger with him now.
So it might honestly be best to call your local police station for some honest advice about the methods he is using to try and get it all back in one go.