Making Sure She's Safe

I rarely interfere with my wife and her lovers about what they do in bed but a little while back I had to put my foot down.  As we were getting ready for bed, I noticed what was clearly a very bright red welt, the imprint of a belt, on her back.  Now let me interject that my wife is not "submissive" and doesn't enjoy spankings or such but as she reaches orgasm, she does like a hard swat on the butt.  It turns out that as she was cumming while they fucked, her lover went to swat her ass with his belt but he was starting to cum also.  He missed her ass and hit her quite hard up on her back.  Since one of my primary duties is to keep her safe, I had to insist no more belts.  If she wants a smack by hand, OK but not with a belt.  She knew I was a little pissed and she communicated my displeasure to him.  She told me a few days later he promised only to use the palm of his hand in the future.  If it had happened again, I would have told her I didn't want her seeing him any longer but thankfully it didn't.  Mainly because I don't know if she would have actually stopped seeing him or just done it behind my back.