My bestfriend is getting bullied and I'm scared for her!

My best friend of four years is getting bullied and I don't know what to do. She's in her second year of engineering college. We don't attend the same college, and she doesn't have any friends there. She had one friend but unfortunately, that girl turned out to be a piece of crap. My best friend already has mental health issues, and I'm afraid this is going to push her over the hill.

We're going to call that girl B for this. So, my best friend and B became friends at the beginning of their first year. They're in the same class. B was always eccentric and acted like the world revolved around her, and she didn't know how the world worked. My friend thought it was just because B was overly pampered by her family. B is one of those people who cry and run away because a cat chased her and ordered boys to carry her backpack for her.

So, the main issue started last semester when B asked my best friend for notes. My friend gave her the notes she had but B asked for three more topics' notes. My best friend told her she hadn't made notes on those topics yet and to make her own, reading the textbook would help. B had some notes from a senior; my best friend asked for them, but B refused. This happened on WhatsApp chat. B went around telling people my best friend is a b*tch and refused to give her notes. My best friend confronted B and asked her why she was doing what she was doing, and she simply said, "I didn't like your tone when you said that. It was rude" My friend asked her how she knew her tone was rude when they were chatting. When my friend suggested breaking off the friendship B got all defensive and then desperate. B told my best friend to simply explain things to her because she's slow and takes time to understand things.

That was months ago. B is still going around telling people my friend is a bad person and getting all of her friends to berate and bully my friend. I was seriously contemplating going to her college and giving her a lesson but this could cause legal issues as we're all adults, and also more problems for my best friend as B is Richie rich and is friends with most of the seniors. Even their class teacher is with B and targets my best friend for things that everyone does.

I need some suggestions as to how to make things easier for my best friend. I can't lose her.

SUMMARY: My best friend's classmate turned everyone against her with lies and now all of her friends are bullying my best friend. One of the professors, the class teacher, also targets my friend for the most mundane things and sometimes for no reason. My friend has no friends there and I don't know how to help her. Her mental health is already bad and all this bullying is making things worse