oh boy, this is a loaded gun question
Depends on the person...so I'll speak for myself here...
Darlin, at my age, if I thought someone special enough to share life with him...as an us...and he cheated on me...I would leave him...without a doubt...
Two wrongs do not make a right, actually what he did, does not give her permission to do it, as well....it is deception, one of which there is no excuse for...and personally, it would be a great infidelity to self...
If your asking for a reason to approve this type of behavior, there is none.
But, this is my point of view...
Sometimes two people can work things out...maturely and with a great deal of patience and understanding....I've seen it done....people do make mistakes, they don't mean, and by God would never go there again...so, it depends on the two people involved....and I'm not advising anyone to leave their spouse...just vocalizing that I cannot deal with infidelity nor breaking trust. Not at my age. Smiles and hugs