I'm going to get in touch with my friends soon to be fiance and tell her that he is using her to free his family of their debt
Do you think it's a good idea to contact her?
Yeah, I think she has a right to know.
I think it's great that you want to protect your friend, and I'm not going to talk you out of it, but I'd suggest that you go about it kindly. Getting involved in the relationships of others can backfire. "Don't shoot the messenger" is an idiom for a reason.
I still think she needs to know.
You're right that it might be good for her to know, but I don't think it's your place to tell her. It isn't good to get involved because it will probably just end up hurting your relationship with your friend.