I dunno if I would agree with the document advice. From a legal perspective, the document wouldn't do much of anything. There's no "consideration" that would make the promise to pay for the surgery a binding contract in the future, and if it's a simple gift the act of conveyance would be enough to complete the gift, no paper necessary. The document might be useful as evidence of some sort of promissory estoppel, but that's a dodgy bet at best.
Basically, all the legal document is going to do is add a level of formality to an already fraught situation. I'm not sure if you should accept the offer, or if I would in your place, but I do know that if you do want to, you should do it because you trust him and the situation, and an attempt to formalize the arrangement would vitiate the trust.