How do you handle loved ones' mistakes?

A person I know is going down a bad path with alcohol, and I’m really concerned. Yet, this person is not my child, and they are a legal adult, so I can’t really make them stop. (They are not driving drunk, and even if they were we don’t live in the same city so I wouldn’t know about it anyway).

I know you have to let people live their own lives, but it’s killing me seeing this person self-medicate. They used to drink less than once a month, and rarely to the point of excess. Now they are drinking at least once a week (again, that I know about, I suspect it’s happening more) which wouldn’t be a problem if they were not getting completely drunk (to the point of puking, not remembering what happened the night before, etc).

I know some people might think that is just normal, but they are nearly 30, and I know they are struggling mentally. I try to be someone they can talk to about the depression and such, but again I can’t make them do anything about it.

So how do you stop going crazy over stuff like this, when you have no authority to make decisions for them. I’m finding it difficult to just let them live how they want and not care about the consequences.