Ok....Well, Misery loves company.
LOL Anyway, my advice is the same. The reason I asked, is this. The person I'm dealing with here, is one of my ex-students. It would be kinda weird and may change the relationship between us if she knew about this side of my wife and I's relationship. LOL Maybe not, but maybe.
Anyway, on to the advice. I think you know what you need to do here. This is your heart you're playing with. And apparently, she couldn't care less WHO's heart she's playing with. She is playing wiht YOUR heart.....her BOYFRIENDS heart....and her boyfriend's WIFE's heart. And, as her significant other, you are guilty by association. Not fair to you, but I think you get the piocture. So now YOU are ALSO playing with your wife's boyfriend' wife's heart. Wow...Thats hard to follow. LOL Anyway...Tread lightly.
Your wife is playing a hairied game here. One thats not fair to ANYONE else. I think you would be best to simply cut ties now and gently explain to her that you are not willing to help her to help her boyfriend to cheat on his wife. Also explain that if his wife KNEW about the relationship, it wouldn't be so bad. But that as it is, she is the other woman. This speaks volumes about her morality....and if she's willing to help HIM cheat.....Then she likely has the ability to cheat on you. If ANYONE in the relationship dynamic is lacking in the knowledge of any other person, then they are cheating.
Example: My wife and I are married and in a committed relationship. If you and I start to date without my wife knowing, then I am cheating. If, however, I let my wife know that you and I wish to date, and she is OK with it, then we are in a poly relationship....Now, if you and I become committed, and I am also committed to my wife, then you decide to date someone else........without letting ME know, but letting my WIFE know....Then you are cheating, and if my wife doesn't tell me, she is assisting you and therefore just as bad as you.
Do you follow this all so far? So, your wife is just as bad as her boyfriend right now because she is the other woman that he is cheating with. She needs to tell him that there will be no more unless he lets his wife in on the whole thing. If she refuses to do this, then you need to drop her and move on.
This is, of course, just my opinion.