Should I let her?

[Kharnage]

So my girlfriend and I live somewhat far away and on some days I can't hang out so she would hang out around her neighborhood. That's fine and all, but before I and her went out, this guy liked her for a while and she liked him a little bit too. The problem is, the guy lives in her neighborhood. The guy now has a girlfriend and according to her "he loves her very much," so should I tell her not to hang out with him or should I just let them be with their friends. I don't want to sound too over-protective or the jealous kind, but I don't want to lose her. What should I do?