If your concern is about her mental illness? She seems to be functioning at a level where, even if she does have a mental illness, it isn’t at the point where intervention against her wishes is appropriate. She’s getting around, making friends (and losing some of them). Her personality sucks, but some people are just like that. There’s also the issue that, with ‘old school Russian parents’, there’s some subtle cultural differences going that are influencing the way she talks and behaves.
She isn’t a psychopath. Nope. She isn’t. Does she have Anti-Social Personality Disorder (which I think is what you’re getting at?) Possibly. But, more liky, she’s just a jerk. Does she have psychosis? Not based on anything you’ve written. Is she autistic? Well, now we’re just clutching at straws.
If you can put aside all the labels for a moment (because none of us have sufficient qualifications and personal knowledge of this person to use them accurately in this situation): can you flesh out what it is about her shitty personality that’s confusing you?
As a self-described empath’ (sigh - labels!), is it possibly that you can’t get your head around how someone can treat people like this, and be okay with that? Is that the issue? If that’s the issue, it’s not about whether she’s mentally ill - it’s about wrapping our head around the concept that some people are just really horrible, and how does that happen? How are they okay with that? Why are they like that? It doesn’t make sense.
The answer, in most cases? Is a complicated mix of personality, genetics, environment, upbringing, life-experiences, and on and on. The key is to identify “I don’t like this person”, and remove them from your life. Pathologising people that you simply don’t like or understand? Is not the issue, it’s a distraction.