Yep, that's cuck mentality 101. Sure, there would be some shock and surprise, dude would probably feel numb to be honest, but then shortly after, within a few seconds one of a few options would happen:
1) Dude pull out his phone and record it, not for sex but to make the divorce/break-up final and easy
2) Storm into the room causing holy hell and ripping the other guys junk off (or trying)
3) Quietly back up, grab what ever's valuable in the house that he wants and leave. Immediately freezing all bank accounts/transferring money from any joint account into his, freeze credit cards and have new one sent to new address, etc.
If the guy was smart they'd do option 1 if married, then probably option 3.