i think some of the reply's here are very one sided yeah he's a lier and a cheat but , wouldnt most of us like to have a little extra on the side ?? and just because he wasnt with just one woman it duznt mean he loved her any less or only did it to look big yes i agree your sister is hurting now but she isnt gna die from it m8 and he obviously wasnt beating her which would be much worse ..
@ the end of the day m8 i dont see why it should effect your friendship with him as long as he knows you arent happy about the situation .