I can't form an opinion, because you haven't given us enough information. What were the circumstances? Who was it with? Did he care about her, or was it a fuck? Was it once or many times?

...and even if what he did was very wrong, one of my least favourite phrases is "Once a cheater, always a cheater." That is complete and utter BULLSHIT. For some people, yes; others grow and evolve, or had a momentary lapse.

Also, don't forget, monogomy is biologically unnatural for men, and instincts must be fought to conform to the society ideal of a relationship.