Hang on a sec... what's the problem with showing her the texts while he is there? She's going to have to tear him a new one at some point after she finds out... just show them to her, whether or not he's there... heck, if he IS there, she can check his phone at the same time and double-confirm that they came from him.
I really hope this doesn't come off the wrong way, but... you owe it to your friend not to stall. At the end of the day the guy is a slimeball... she needs to know sooner rather than later so that, even if nothing else, she can go and get herself tested for STDs. Because while it's great that YOU'RE shunning him* other women might not be.
It's more than an emotional issue and I don't know whether or not that was brought up in the other thread. Her health could very well be at risk, here, now and in the future. I know you don't want that to happen to your friend at all, and it would eat you up inside if she ended up catching something from him while you were talking to us and feeling squirmy about blowing the lid off of the thing.
I'm sorry this asshat has put you in this position, and I'm triply sorry for his poor wife.
I just hate people sometimes.