Hmm...a doctor who says six beers a day is A-okay huh? Perhaps he also has doctor friends named Jim Beam or Jack Daniels. Just kidding. I just can't imagine a doctor giving the okay for the six beers a day either.
Look...it sounds like you are very worried about your friend. You know and I know that the beer drinking is probably not the best thing for her to be doing but I honestly don't think you can scare her or inform her out of her actions. Mostly likely she is attempting to self medicate her symptoms and/or depression.
But your question is what does drinking do to a person who has MS. I didn't have to search long to find a forum thread where people talk about this very thing. Here is that thread where some people with MS talk about developing an aversion to alcohol as it makes them feel worse.
I also found an article discussing a case study of alcohol abuse in a patient who has MS.
From what I have read, it seems that alcohol does exacerbate MS symptoms. I mean...you already can have issues with gait, balance, speech, and so on and if you add alcohol...it can be dangerous. I have also heard that it is easier to get drunk faster when you have MS.
I am thinking that if your friend could get a good doctor who would treat her MS symptoms and including any depression if she has it...that she wouldn't need the six beers a day self treatment. As a friend you can tell her these things and say that you are really concerned about her. But you are right that it ultimately will be up to her as to what she chooses to do from now on.
In my humble opinion....she could get hurt or injured if she is intoxicated and has MS symptoms. I am not drinking and I fall down with my MS (see my latest video post).
You are a good and caring friend. Let's hope your friend listens to what you have to tell her.
Good luck and tells us how things go.