I'd focus on the husband. Maintain dignity at all times. If the OW does know about DSis then maintaining dignity will seriously wind her up. Not every OW knows though...
I found out my ex-H was seeing someone else when the OW rang our house one evening (before the days of mobile phones).
She asked to speak to ExH and, of course, I said he wasn't in, could I take a message. "Yes, could you tell him I'll be able to do the weekend after next after all. He'll know what I mean". "OK...sorry, who are you?" says I. "I'm his girlfriend, X. You must be LondonJax. He's told me all about you".
Well you can imagine what went through my head - I don't think I've had so many thoughts in so short a time! It turned out that he'd told her that he shared a house with a woman called LondonJax (so he had the excuse that he couldn't bring her back to his home). What he hadn't told her was that the woman he shared with was his wife...we had a very interesting conversation when I put her straight.
Mind you, my ExH was a nasty piece of work who'd walked all over me for years so he probably assumed he'd get away with anything.
ExH had a pretty rotten year that year. I got the hell out of that relationship and the OW dumped him immediately. I, on the other hand, got a very good divorce lawyer, got a nice little flat and a promotion at work. Then, a few years later, met my DH, married and had DS. Last I heard ExH had remarried - good luck to that poor soul as he tried to text me to arrange a drink without her knowing! I showed DH that one and blocked ExH number. Leopards don't change their spots...