He's that dude who doesn't have any dating game on his own, so he hangs around couples waiting for the breakup so he can first be "the friend" to the girl, with hopes of it progressing to something more. He's always trying to stay friendly with his friends' exs. It sometimes works, but usually also breaks the bro code and leads to loss of friends, trust, and respect.
Rick: Who's she dating now?
Andy: Remember that little relationship lurker Seth she was always bringing along when we went out? They hooked up.