What defines Cheating?

There has been some major drama in my group of friends the last week.
My close friend Michelle has been dating a guy named Rob for 6 years. She was very much in
love with him, and was patiently awaiting the day he would ask her to marry him. Now, Michelle got transfered to another restaurant in January, so her concentration was pretty well focused on her new position, and her busy schedule for a time. Her boyfriend was himself pretty busy with work, and they didn't get to spend as much time together as she would have liked. Rob, however, decided he needed to do some socializing, and began hanging out with one of our friends, Heather. They went out for coffee and to the movies and for the occasional dinner, what else, i'm not sure, because i'm not them, but michelle tried to see it as just her good friend hanging out with her boyfriend, and nothing else. And maybe thats all it was then.

The past month, however, has seen a strain on Rob and Michelle's relationship. He became extremely distant, and when he did talk to her, it was to argue and bicker about things. Rob began to do weird things, like get a separate bank account from michelle, and go long hours in her presence without speaking a word to her. She just figured he needed some space, and let it go. But after a few days of this, she could no longer stand the pain he was causing her. She confronted him, and asked him what was going on. In a quick, blunt, conversation, he told her she was not the person he wanted to marry or be the mother of his children, he wasn't inlove with her, and that he was done, and their relationship was done.
Michelle took this pretty hard, and she phoned her close friends for support. We all responded immediately, trying to help her get through this. Except one of us. Heather. Michelle kinda got nervous at this fact, but decided that she just may be busy. A friend and I looked at each other upon hearing this, and knew, something just wasn't right.

Finally, with nothing back from rob or heather, michelle confronted them both. Aparantly they love each other, and have been seeing each other for weeks and saying they loved each other. But because they did nothing physical, rob didn't cheat on michelle, and heather didn't steal her boyfriend, according to them. They expect michelle to "get over it" and just "accept" them. They see nothing wrong with the fact that they were exchanging love e-mails while rob and michelle were away at his cousins wedding. Ya, he took her to a family event a week before smashing her world to pieces. While she was thinking, this will be us soon, he was thinking, how soon can i get to my laptop to e-mail heather back? And this is the biggest kicker. Heather is mad at michelle for telling our friends what happened. She says now she feels like everyone hates her, and she gets stressed at work. What, thats it? well i guess we're not doing our job as friends, maybe i should be egging your house and shredding the tires on your car for what you're doing to MY friend, you evil, two faced..... relationship wrecker......well, thats just how i feel. But what do you think?

If you EMOTIONALLY cheat on someone, isnt' that cheating? is that not being unfaithful, when you're sharing you're love and feelings with someone other than your girlfriend for the past 6 years??

I'd really like to know, so I can tell Heather exactly why none of us would like to be her friend right now, because she ripped michelles' heart out of her chest and took a nice big juicy bite, and then cries, I ain't no canabal!