I get the wanting proof to confront, but I can only say from my experience, even with evidence they both still lied and twisted things - I’d been asking for months if something was going on (ex and my best friend) they made me feel I was going mad.
I saw some text messages (not proud of snooping but ‘needed proof’ he deleted a lot so nothing concrete in the texts but still inappropriate) and they still lied but I asked him to leave. He did. For another month they both declared their innocence until it finally came out (help from mutual friends who had been used as alibis unknowingly).
I completely understand your fear, they make you think it’s all in your head, but honestly, you know the truth deep down, you don’t need the evidence to end things but I get why you want the proof and that it will help your argument.
im so sorry youre going through this, its awful. As for telling the affair partners husband, it’s tricky. Having been cheated on, I would 100% want to be told, but I’ve seen it backfire for others. I would maybe suggest she tell her husband and that you will not lie for her. Do you know the husband well? That would sway the option for me, if I knew him, I’d have to say I think. Or maybe tell him your suspicions, it’s up to him if he takes that further.
good luck to you, it’s not easy navigating something like this (I’ll admit I was an absolute mess and i didn’t handle things best I could in retrospect) but you will be happier in the end. I almost felt relief once I had ‘proof’ and admissions because that gut feeling mixed with their deceit, gaslighting and denial is a tough bloody gig!
you will get through it and settle in to your new normal, lots of hugs