It's a reminder that the brain is never truly satisfied when gambling, there's always the desire for more.
And not only while gambling can such scenario be reference with, because it is human's nature to have an insatiable needs (i.e according to economics), whereby the more we get, the more we want to get more, which is obviously what happened in this scenario with you and your friend.
I believe that friend deserves an advice, but I don’t want to embarrass him myself,
I literally don't think he deserves any advice, apart from the aspect of letting him how it's totally wrong to make promises and failed to fulfill it. Just as he told O.P he will be giving him 10% if he wins, and he won and never fulfilled his promises.
is there a good way to talk to him without making him feel that Im indirectly asking him for the money he promised me?
If I was to be you, the best I could do is "Let Go", and act as if nothing ever happened, because for the fact that he promised and failed to fulfill his promises and not trying to afford me due to guilty conscience, the best I could do will be to behave as if nothing ever happened, and then be fully conscious whenever I'm around him, knowing fully well he can never be trusted with monetary agreement.