My friend and I are having a joint 17th birthday.?

If she's really your friend then you should just be straight with her. You don't have to be rude or anything, but tell both of them that you have a bad past with this guy and don't want him at your joint party. If need be (in case they're stubborn) point out that this party is meant for the both of you and you should both feel comfortable. Try not to bring up the money issue with your friend or she might think that what you're really upset about is the financial situation. Good luck, and don't be afraid to stick up for yourself.