The sleeping with a married man is just so completely not right. I say what goes around comes around.. karma and all... someday when she's in a serious relationship, that will be one of the last things she'd (or anyone) would want to happen to her.
The other issue? that's her perogative.. at least apparently the guy is open with her about his condition.
But hey, if we have to keep silent to a BFF when they share such important things? That's ridiculous. Truly, that is what a close friend is for-- you don't have to wait to share your opinions about stuff, or fear you'll ruin the relationship if you have a differing opinion...ou're too close for that, if truly you are BF's.
And someday, either she'll thank you for telling her the truth about how you feel (and be wishing she'd have heeded your advice) OR you will have grown apart by then....
Best of luck to you.. she does appreciate your input, or else she'd be hiding this info from you. I'd just lay low and wait for her to take the lead... and make sure she knows that you care no matter what she decides to do and you support her no matter what.