What Do You Consider Cheating?

This question means different things to different people. Most people in traditional monogamous relationships believe sexual infidelity is cheating. Others believe it's deeper than just sex.

What if you're in a relationship and find yourself connecting with another person? Can you get all things you need from one person? Different people bring out different qualities of your personality.  But when you are in a committed relationship, should you give up these connections with other people? 

Is it cheating if you have a mental connection with someone? Could this be the definition of Friendship or is this an aspect of Cheating?

I have a friend who doesn't realize she is in a intimate relationship with a married man. Don't misunderstand me... they do not have a physical relationship. In fact, their interaction consists of strictly phone calls and texts. Their conversations are platonic in nature, no sexual banter.

But he shares his fears, dreams and frustrations with her, the things you would expect him to share with his wife. I've witness them talk for an hour with ease. He reaches out to her when he feels defeated, sad or even elated. He calls her every night to talk about his day. Her voice is the last thing he hears before he goes to sleep.

She is obviously filling a void in this man's life and he fills one in her's. She refers to him as a friend, but would his wife have the same understanding? Are they just friends or are the lines of friendship blurred? Is this cheating?