In a "woah, calm down" moment, can I put in an aside?
I have a number of friends who are professional poker players. They regularly will blow £20k in an evening, and then have another evening where they ar up £40k. The skill is in knowing when to extract money from the poker pot and move it into (not to be touched for poker) real bank accounts as a salary.
It is very difficult for non-gaming outsiders to see, but gaming money and real money should not be seen as the same thing, think of one as tokens and the other as the stuff that pays the mortgage.
I would suggest, if it hasn't already been done, separating the accounts which he may use for gambling away from the accounts which are for real life and if he wants to gamble he may only use the (debit only, no overdraft) former to do so. As long as only money from that account is used and no promises are made, debt is impossible.
If he ever takes from the real account to fund the other, that's when you can react with the whole "addicted, problem, alarm" type responses, but it is perfectly possible to make a good healthy living from online gambling without it becoming negative.
Also, I wouldn't find an 8 hour gambling session out of order if he wants to be good at it, after all, you'd spend 8 hours at work wouldn't you?
I'm not saying he isn't addicted and doesn't need help if that's the case, but there's no need to cry foul if that isn't the case.