I think if you decide to talk to them about your concern for them it'd be wise to prepare yourself for disappointment. Hate to sound pessimistic, but people with drinking problems who actively drink tend to resist any help or even the suggestion they need it.
Don't know your friend of course, and their personality. If it were you talking to me when I was active in addiction, I would have: suggested you were a worse drinker and had no right to judge, that I'm having a good time so there isn't a problem, insisted you acknowledge the two days this week I didn't drink... and so on.
I have a friend who I'm now thousands of miles from. It's tough, but I'm seeing all the signs that she's slipping into problems. E.g., she'll post on Twitter that she's drinking a bottle of wine in the bathroom because her husband's upset with some behavior of hers then gloat about how "awesome" she is.
My response so far has been not to acknowledge it at all. If she starts to express regret or guilt, I might step in with a "how are you doing?" Otherwise I'm just the wet blanket trying to ruin the "party."