How do I explain to my friend that she has changed, and not for the better?

Your friendship has become toxic. I would cut her off. If she is talking nasty about others all the time, and behaving like you said you need to ask yourself if you need someone in your life like this. Most likely she talks the same way about you, behind your back. If she loses her friends it will be her own fault. She is an alcoholic and needs help-but has to admit she has a problem.