Not quite the same situation as yours, but something similar. My wife had been going on about this guy who worked in her building, the business next door. Apparently they'd been taking breaks on the building patio more than once. He'd bought her lunch a few times from the food truck and they'd gotten quite friendly. She said it was all quite harmless, but I thought otherwise.
I arranged to run into him one afternoon when she'd already left work. I introduced myself, we chatted a bit. The I said "Are you trying to get in my wife's panties?" He was quick to deny anything, but I cut him off with, "Its okay if you are. She's pretty hot, so I understand...". He was trying ti decide if I was serious, so I handed him a stack of my wife's nude and semi-nude pix. He said" Wow, its not fair that you show her to me like this, unless you're serious." I assured him I was. I said he was welcome to try, but I doubted she'd really cheat on me. I told him she'd likely get excited though, and come running home eager to fuck me, so it was to my benefit if he came on to her.
I knew after seeing her pix he'd not let it go, and he didn't. But I was wrong about her not yielding to the temptation. Within a week or so he let me know they'd been to the motel a few miles away from work, on their lunch break. And he'd fucked her brains out. AND...she never even hinted that there was anything going on. This went on for about 2 months, until she felt too guilty and broke it off.