The word destroy for gamblers is subjective, he might not be aware of it and takes it something that normally happens to any gambler so he will not accept any kind reasoning, as long as he can balance his winnings and his losses, I don't think he can admit that he needs help, but when he is not winning and losing everything, maybe that's the time that he will realize that he needs help.
Its not surprising and its pretty common on where people do only realize that they are doing something wrong when they dont already have the money to spent on gambling.
reality talk mate,because many gamblers can even spend all their money and even everything and not noticing that until time comes that they have nothing to spend and will ask for forgiveness .
Leaving in mid way is almost impossible specially you know that you do still have money to spent then you would continue no matter what or does simply the party does continue.lol
Leaving gambling or telling people for them to stop is useless.Just let them be and experience would really be a great teacher on most cases.
and just stay by their side,be a comfort place whenever they need so realization may come earlier before everything loses .
sometimes Gamblers only needs company to lean on,because most of them are Losers in life and only in gambling they can find superiority .