Too bad for your friend!!! Someone forced her to have sex.....yeah, like I believe that one.
She cheated on her husband. It's time that she fesses up. Get it out in the open and deal with the consequences of her actions.
Just a side note for you....
during my trials and tribulations I have come to realize that most women are not wired the way men are. It's not likely that a happily married women will have a fling. I'm guessing that your friends marriage is already in the toilet.
Sorry to be so frank, but it's the truth.