I'm Lucy, and I'd be happy to answer your questions today.
If you sign something stating that you are giving them a gift, they have zero legal obligation to repay you. Gifts are forever and cannot be revoked. Only a loan needs to be repaid. When you loan someone money to buy a house, you can have them sign a piece of paper giving you a lien on the house, which you would then record - that stops them from selling the house, and gives you the right to sue if payments are missed. You would also be able to stipulate that if they break up, the loan becomes due and payable in full within, say, 90 days (to give them time to find a buyer) or some other amount of time you're comfortable with. That's the way for you to protect your investment. But if you want to be able to put a lien on the house, the girlfriend has to sign the paperwork. If only your son signs it, only he has to repay you.
Also, if the girlfriend is the only one on the mortgage, she's probably also the only one on the deed. And that means your son has NO protection if they break up. His girlfriend will be able to ask him to move out by giving him 30 days notice and she doesn't have to give him back any of the money he paid in. So it's in his best interests for the two of them to see a local attorney and have that person write up a contract that will protect him in the event that they split up.
If you have any questions or concerns about my response, please reply WITHOUT RATING. It's important that you are 100% satisfied with my courtesy and professionalism. Otherwise, please rate my service positively so I am paid for the time I spend answering questions. If you are on a mobile device, you may need to scroll to the right. There is no charge for follow-up questions. Thank you.