Hey, I'm 21F and I'm giving advice to you like I would to my little brother

First is, don't go telling her to her face that you think she is lying. Even if she is or isn't lying, she'll say that she isn't lying. So it's up to you to think about it and come to a conclusion yourself.

And maybe you should put distance between you and her too, regardless if she actually has it or not. You've known her for only a few months, you never met her in person. Your options are to cut her out or to watch your best friend die. Which one do you prefer?

I think all this person will bring is pain for you, and the sooner you cut her out, the less it'll be better for you. There will be a lot of great people who will cross paths with you so don't worry about not finding anyone special.