I think itdepends on how she acts with him and you afterwards.Mygf slept with my best friend but after it was over he kept on hitting on her and she ignored him and was all over me,thats good thing.If your gf is still flirting with him afterwards and talks about him when he's gone or even chats...
My girlfriend has a very old house that needs plumbing bad and asked me to ask my plumber she would fuck him if he did some work forfree,true story...i was shocked!