Don't tolerate your friend by lending him money. It's going to be your daily problem if you have allowed that. But that's good that you have never given him any loan for him to gamble. Well, at least for him he's honest that he told you where he'll use the money. It's rare to see gamblers telling you the actual reason why they're borrowing money.
You're correct, many people I know they ask money with a reason for emergency needs or daily needs, this make people willing to give money because it's an important matter. Even users in this forum are asking money with a reason "personal", but I know there was someone who said that actually use the money for gambling.
It's a simple thing, if someone is really good in gambling, there's no way they will ask for money because they can multiply their own money.
Yeah, you cannot be sure if they're telling the truth about the emergencies. But as someone who worries with others, you're for sure going to think that it's a genuine reason. Unless that person is a known gambler, you'd know that they're telling a lie and they're not going to give the actual reason for them to be able to successfully borrow. In real life, this really happens and I've got relatives that have done this but to no avail when our other relatives have learned their addiction, they're no longer helped by them and to avoid being an accomplished to their addiction.