One thing that can help is to let go of the OM yourself...calling him names (even if they fit) keep you in a negative frame of mind...that hurts you and eventually can hurt those around you...
It will take time but if she does want to work on the marriage she will eventually be able to let him go...I have a man that I got attached to many years ago when my I found out my husband was cheating online (EA)...when my H realized how much hurt this was going to cause (I found out as it was ending) he wanted me to give this friend that I found...there was nothing ever romantic between us...he was a sounding board and kept encouraging me...really a nice guy that helped me relate to my H better....well, H didn't approve because of how he got snagged and forbid me to have any contact...this was hard...then one day H said he didn't care...I resumed contact...and when H left me he was one of the first people I called...still a friend to me...and I still keep in touch...although it is not often at all...just to touch bases and see what is up in his part of the world vs my part...and then there was one man that I became romantically interested in while H was gone...again H thinks I never talk to this guy anymore...I wouldn't if there were any romantic feelings between us but there isn't...and again it is mostly "Hi, how are things going....I am doing this or went here...bla bla bla."...I would not lie if H asked me about either one...but I don't flaunt it in his face either...I have nothing to hide...but I wouldn't want him reading my emails either...
What it boils down to...is you can't control her...but you can help bring her back into a close relationship with you and thus the OM will become insignificant...and EVEN IF she keeps contact...it doesn't mean that there is anything there...to me the fact that he is now going for her sister, whom she introduced to him...would tell me that she really isn't all that hung up on him...but it might be more about knowing he is forbidden by you????
