If the other couple's relationship is already taking strain, and this guy has cheated on his wife etc., then maybe there may not be enough of a "store" of good will for it to pan out for them. Maybe it will be the last straw for her. Especially since you say that it seems to him more of a "fucking around" thing than anything else.
But on the other hand it could be exactly the catalyst needed to start those important discussions and help them both move towards something a little more open etc. Maybe you could encourage some communication within their family without pushing yourself too much into the dynamic? Encourage him to speak to his wife etc (since Curly will insist on that anyway). I can imagine that, in the headspace of the wife, she might not appreciate the intrusion and could feel ganged up on if it's essentially you three against her if she decides she is not open to the idea.
Its been my experience up till now that (unfortunately) some people are hostile towards learning how your relationships work, specifically for the reason that it forces a sort of introspection that may be too uncomfortable at that moment. Seems to me that people make that progression in their relationships, and lives in general, at their own rate and although you can be an inspirational force, it really needs to come from them.
Just my waffly two cents