How to make friends from your enemies?

My cousin asked me this question but I couldn’t find a good answer for it so I thought maybe you can help. She specifically asked how to make friends in general and how to turn kids in her class who dislike her, or so she thinks and actually she considers them her mortal enemies, to be her friends (she’s in grade 10 by the way)
She was an outgoing person when she was little but some family events involving a stepfather and problems with her grandparents turned her into an introverted person and I think that, since she had no brothers or sisters and that she had to carry that burden on her own, that made her become that way.
She told me that she tries hard to talk with the other kids but she finds talking stressful and so she remains silent when a conversation is going on. And when she tries to participate in the conversation, she doesn’t know what to say except for a few comments if possible. Eventually she ends up sitting alone and so she hides in any empty class she finds.
I am not sure but she talks to me and other members of her family, so I don’t think it’s a psychological problem. She’s even smart and beautiful…
I know this maybe too much information but I really want to help her, so anybody has any ideas?