It's pretty easy to get the main chick to accept a side piece, especially if you treatin' her right and slangin' good D on a nightly basis. It's HARD to get a side piece to accept that she is just that, unless there is a condition that she is forced to accept (e.g. you were already married, she doesn't live where you stay, she's way younger/older than you, etc) I've had both sides know in general they wasn't the only one, but for them to know each other and be cool about it? You mean like "Sorry babe, I was over at my main chick's house"?

OP's friend's pimp game tougher than teflon
