Wayne and Wanda, you're off target on this one. 
1) He did it behind her back. If it was innocent, he would have been doing everything in the open.  Been there myself and know it had nothing to do with cheating but everything to do with knowing my wife wouldn't like it. I knew my wife wouldn't appreciate me in contact with an ex. I also knew I had no interest in ever having any kind of intimate relationship with her either. She was having issues with her teen daughter and I was giving advice on how to deal with it. This was all done on our high school alumni website. My wife, after cooling down, explained to me that by not being open about it showed I cared more about myself than if I had told her up front. In high school(40 yrs ago) I cheated on my wife (then girlfriend) with this person. 

I was being selfish even though there was no intent on being intimate. My need to help was greater than my desire not to hurt my wife. Was it cheating, in a way yes. It cheated her out of her trust