Serves the idiot right!
I went through something similar recently with a guy on my course. He became increasingly stalkerish, texting and calling at all hours, following me around campus so that he could try and catch me alone etc etc. I realised that he never approached me if I wasn’t alone, so I made sure someone was with me all the time. This worked but it wasn’t fair, I had the right to walk around campus on my own without the fear of some idiot following me/harassing me.
So one day I approached him (he was stunned as well), sat him down and explained to him how uncomfortable he was making me feel, how I would report him if his behaviour continued and how I knew people who would “sort him out” if he continued the way he did. It’s actually worked so far…no more texts, no more phone calls, nothing. I was quite proud of myself actually, dealing with the situation on my own 
Hopefully it works for your friend, these guys deserve a swift kick where the sun don’t shine 