I don't think a specific reason is necessary for someone to start gambling again. Often, just a momentary urge or seeing someone else playing and having fun can spark the desire to try their luck again. It doesn't have to be anything grand: in some cases, simply watching someone else gamble is enough to rekindle that desire.
If his reason for returning to gambling is to have fun and he has no intention of chasing the win, that will be okay. He knows that gambling can put his life in danger and because of his experience before, he needs to be careful and not spend too much money.
If he has good self-control, he will not intend to return to gambling although he is watching someone else gamble. He will feel enough to gamble before and knowing the risks of playing gambling. So he will not return to gambling and just watching someone gamble.