I skimmed the rest of the replies...and there are some really smart folks here and you should consider what they have to say...but I did want to share my "take" before I went to bed.
First off, this is one of the few areas where I disagree with my husband (and many of the folks here). It is my position that the person who is actually IN the relationship (such as the relationship between your M and her husband) bears the PREDOMINANT responsibility to maintaining the boundaries of that relationship.
Yes, if you truly care about(/love) her then you will try to help her by enforcing any decisions that SHE makes about what is right for her and her relationships (i.e. If she asks you not to talk to her/ communicate with her because it is difficult for her to not respond - then, if you care about HER, you will help he stick by her resolve.)
I have shared my own experiences here (you can read about it in my "Journey" blog - the /jackassery/ section). When I broke the, albeit ill-defined, boundaries with my spouse, an explosion occurred. Part of the outcome of that was a "no contact" demand - which I respected as well as I could. If Dude called...I didn't answer, but told MrS that it had happened. If I ran in to Dude, I let MrS know the specifics of the the encounter. If Dude emailed, I deleted it without responding and let MrS know that he had contacted me.
10 WEEKS later...Dude emailed MrS with an impassioned plea for forgiveness. (Which was pretty awesome, if you ask me, since I was the asshole in this story.) Some stuff happened...and...happily ever after (so far - 2 years later).
Now, my situation is (as all situations are) different. MrS and Dude were "best friends" before all this went down. BUT, I am actually convinced that it was my dedication to enforcing the "no contact" dictum that convinced my husband that I was sincere in my desire to doing "whatever necessary" to preserve our relationship. So, when Dude contacted him, he was already convinced of MY dedication. (OK, 10 weeks was actually well short of my time frame - I had told myself that if MrS was still in turmoil at the "1 year" mark then I would "let" him divorce me without a fight.)
My point, if I have one, is that THEY have to figure out THEIR relationship in light of this, new, information. And that takes TIME. Yes, your wife is awesome - granted. But this may be an entirely new paradigm for him - if you (e.g. you+her) give him time, then he may be able to wrap his head around it, but if you push him, you may shut down any possibility of that.
At some point here, you implied (said?) that "if it didn't work out with her husband" then you could pursue this further. BUT, how much BETTER if things DID work out with her husband AND you could pursue things further? (Tongue-in-cheek: how much MORE poly could that be?)
"Life is short, but the years are long."
"Waiting is, until fullness."
Me: poly bi female, in an "open-but-not-looking" Vee-plus with -
MrS: hetero polyflexible male, live-in husband (21+ yrs)
Dude: hetero poly male, live-in boyfriend (3+ yrs) and MrS's best friend
Lotus: poly bi female, "it's complicated" relationships with Dude/JaneQ/MrS (1+ years)
TT: poly bi male, married to Lotus, FB with JaneQ
VV and MsJ: bi-women with male primaries, LTR LDR FWBs to JaneQ
My poly blogs here:
The Journey of JaneQSmythe
The Notebook of JaneQSmythe