Make sure she is fatter and uglier than your wife. When your wife asks who was better. Say your wife. Even if the other bird could suck a football through a garden hose.
Read Frank Skinners Biography and he mentions when he once had a threesome ,to cut a long story short the two women ended up accidentally nutting each other in a certain act and it all ended up on an anti climax in more ways than one .
It's always the ones you least suspect! Edit: Hope the non-quote is ok. Let me know if your friend wants it editing...