but it’s none of my business really
^ This.
What your mate and your boss’s husband get up to is their affair (no pun intended) – keep your conk out, well and truly. What you don’t know can’t hurt you.
I’ve been in this situation knowing that a good friend’s partner was playing away.
When it all blew up and he found out, he asked me if I knew – I had to say yes at which point he went beswick and asked my why I hadn’t said anything to him.
I replied that it was none of my business – if I had said anything at the time it would probably have caused more grief and ruined more than one friendship. But by keeping schtum (and I know it’s difficult) you may salvage something out of it at a later date rather than ruining everything.
Your work mate may well end up hating you, but your boss (possibly, after the initial wailing and gnashing of teeth and tearing of hair) MAY trust you more because you’ve shown that you can keep silent about confidential matters, and look to you for support. Your bosses estimation of you may go up in that case which can only be a good thing, yes?