Wow! I definitely missed something here but I think I get the just of it. I don't think I could ask for the money. It may seem like it's owed to you somehow or you may just genuinely need the help but it wasn't his parents who took the money from you. It was your abf, the abf that you CHOSE to stay with whether you had high hopes or not; it was always probably in the back of your mind what he was doing with the money but you kept giving because you cared. No fault in that.
My exagf is partly responsible for getting my parents' van stolen. But I'm partly responsible too because I loaned it to her in the first place which I never should have done for more reasons than one. Would I like some kind of reimbursement for that? Yes. But I wouldn't think to ask her parents to pay for it. However, I'm constantly reminding her that she's going to have to make amends somehow and of course, I realize I owe something here too.
No judgement honestly, if you've made up your mind to do this but just be careful that your motives are pure and there's not some other thing that's motivating you like pain, disappointment, or anger. If it's been offered, then go for it but if you're having to ask then keep an open-mind of the results. Good luck.