I don't think it's a good idea if you come across to a random guy or even your friends to give them some advice, but I think it's more beneficial if you help him to go for rehab than just counselling him to help him become a better person.

Giving advice won't help much, but for rehab it would help much to get rid of gambling addiction.
However, if there are no rehab facilities nearby, maybe helping him to go for a vacation far from the casino while counselling him might be a good option to help him to get rid of the addiction.