My friends ex won't stop mailing me on Facebook?

[Sophasaurus]

He sent me a friend request and I thought 'oh, I know him' so I accepted it. And since then he keeps messaging me sometimes asking how I'm doing other times just saying 'hey wassup'. He puts 'xxx' on the end most of the time. Although this sounds really mean he isn't very good looking (spotty, glasses, kinda short)
Also while he was with my friend he would get really angry at her for nothing and called her all kinds of horrible names via text. I know she still likes him a little and its getting awkward because he messages me like 3 times a day and its getting a little stalker-ish. They both go to a different school to me. Should I tell her he won't leave me alone or keep quiet?