Most Helpful Guy
just tell it the way it is. i find it a little unfair that you want to deny him a friendship just because you don't like em, but you have good reason. You don't trust that grubby little cunt... but if you trust your partner... then it should be all good right? just tell em the backstory and to watch out for her.
Most Helpful Girl
I'd tell him the truth. I wouldn't dress it up to try and sound reasonable or anything - I would just tell him that you don't like her because she slept with my ex.