My best friends boyfriend is obsessed with her.?

Saliyah

So my best friend’s boyfriend is obsessed with her. She broke up with him . But she told him that they can still be friends. He is mentally abusive towards her. She recently started cutting herself because of it. He sends her over five paragraphs a day. She said she felt like she was in a trap that she can’t get out of. I don’t know what to tell her .. any advice?