You know personally this might have been a good time for the wife to turn this into a less embarrassing situation by suggesting some sort of participation (even if it's just watching, talking, SOMETHING) that doesn't force her to become involved before she's ready to.

I'm sure this guy's probably got a semi-permanent limp dick from getting caught, when it could have been turned into a growth and acceptance moment for both of them. As it stands it sounds like she inadvertently turned this into a "Oh shit my mom caught me whacking!" moment that probably fills their quiet time with nothing but uncomfortable silence.