Once a cheat always a cheat?

I got a call from my friend today. His girlfriend of the last five years cheated on him. She told him it was just a kiss and he believes her. He is really torn up. He blames himself. He said he had been being an ass and now he is going to go to counseling for his moods. He was actually crying when I talked to him. He lives on the other side of the country, so I couldn't just drop in to visit him and give him a hug. He had told me last week that he ordered the engagement ring he had planned on giving her in January. I know it is only a kiss, but I consider that cheating. For me, I would not get over it and never trust her again. He wants my opinion, but I think he wants me to say he should stick it out. Should he stay? Once a cheat always a cheat? Or should he try to make it work?

The Eagles just lost the first game of the season. I think it is for the best, I could not cope with another super bowl. I was so stressed last year that I thought I would die...