Mortal sin: Grave matter, full knowledge, full consent of the will.
Drinking too much: Yes, not a good thing, can lead to illness, even death, to self and others. Grave matter.
Full knowledge: Most people who drink too much really do not realize how much they drink, or that they have a problem. Probably your friend neither realizes that her drinking is a serious matter, nor how much she is actually drinking, or how she is behaving. Looks like full knowledge is iffy.
Full consent of the will: By definition, someone who is addicted to a substance (like alcohol) has a physical and/or psychological dependence on the substance. While the person may wish to cut down or stop consuming the substance, his or her body or mind may go into withdrawal. IOW, he or she might WANT to stop but be physically or emotionally unable to do so. Looks like full consent of the will to consume the alcohol is likewise iffy.
If all three conditions are not met, the sin is not mortal.
I trust that you have talked to your friend. Gossiping here about her “getting drunk” and being concerned over whether she is in “mortal sin” seems a little, I don’t know, disingenuous.
So. . you know the requirements for a mortal sin. NOW, all you need to do is keep YOURSELF from mortal sin, and PRAY for everybody else.