I can loan him in one condition he needs to be sober and stop gambling for at least a month to show that he is not using the money for gambling and it will be under close monitoring and supervision.
Its hard to grant a loan to anyone who is deep into gambling even if you are close to him or you owe him something, lending him will just give him money and reason to continue gambling in the hope that they can get all their losses back.
This is actually a smart idea, but are you always going to be there to monitor him? As someone that has other things to attend to I don't think you'd have that time to look over someone's shoulders especially when we are talking about an adult here. It's possible that the moment you take your attention from him for a few days or probably even hours he might relapse and pretend like nothing happened