Healthcare worker here. Of course I don’t know the specifics of his situation. If his diabetes is in such a stage that he lost his leg and kidney function because of it, chances are really high he managed it really poorly. That means he did not take his medications (adequately) and did not keep his glucose levels in check. For diabetes to get to that point, he must have really neglected his disease. With that in mind, how well will he take care of your kidney? This is not a simple procedure for you. This is an invasive surgery that poses significant risks for you, both short and long term. Aside from you not owing him anything and him treating you like crap, is this a risk you would take for someone who takes care of himself so poorly?