I had a situation where my friend went behind my back with my g/f I was dating for 3 years... she was piss drunk he wasn't... I also had sex with her that night which makes it even worse ...... That kid was my best friend since I can remember and it's never been the same between us since .... I forgave him but it will never be the same friendship ever again and I barely see him anymore

My advice ... don't do it unless your positive your friend will not care ... this girl is prob just trying to get a revenge fuck on your friend