Everyday my friend gambles with his money as same with his life and he always gambling throwing away his money for nothing but losing and the worth part of it is he owes me money 
If that's the case and he's too serious into it, take him to a responsible authority that might help...
Else, ask him to "do it for fun and don't make it a habit..."
And don't force him to repay you back because in such cases, these guys don't listen, they will try to recover the losses and won't collect the money instead to repay what they owe to someone...