I don't agree with you. This was his co worker. It is in no way "his fault" for leading her there. It was a football party, not an orgy. I believe if this was an important rule, especially because it involves his work, it was her responsibility to obey the rule. I understand cheating and i can see how it can be sometimes forgiven, but this was way over the line. I would be furious and I'm not sure i would forgive it. I am very liberal and open minded when it comes to sex but if it involves my image at work and my livelihood, that's where I draw the same line he did.
Making this his fault is immature and irresponsible. She is showing concern and accountability for her actions and you want to make it about him.
She is an adult not a baby and saying "tisk tisk" is, i'm sorry, IDIOTIC.