He stalks her because it's getting pleasure out of scaring her. Bullies prey on the weak.
She needs to stand up to him and stop taking it. She needs to get a solicitor to get a restraining order; go to self defence classes so she knows how to defend herself. And learn to ignore all the rest.
Next time he hassles her on a bus - two options:
1. A football dive screaming (get him arrested for assaulting her)
2. Turn around and kick him where it hurts.
Whichever fits best.