My husband was blackmail and sleep with the woman that blackmailed him
Wow, I'm sorry your going through this... Must be overwhelming ... Need to know more of the story to be able to give sound advice.
Ditto... although I already suspect it's probably more a case of this:
'No, er, I didn't cheat on you, I only slept with her because she BLACKMAILED ME, yes, that's it, she blackmailed me, meaning I was powerless to say No as well as to fail to get and maintain an er*ction!'
What, like THAT kind of 'blackmail', you mean? ;-)
Otherwise, it's called, Call the Police and report the woman for harrassment or, in fact, RAPE! (- physical knifepoint, psychological knifepoint - what's the difference aside from atoms?). Unless what you're being blackmailed over is your own illegal activities, of course, in which case - welcome to one of your consequences, courtesy of Fate, that you *and* your wife *and* quite possibly all that sail in you, have to suffer!... unless you'd prefer to face your own music to ensure 'the nasty cow' gets forced to face hers?