Some people do pick up and leave Spark People without telling their Spark friends. I know it would be considerate to tell "friends" you're shutting down your page before you do it, but some people prefer making a quick break. Some people feel that if they say they're leaving, their friends might try to talk them out of it. Some people leave SP to go to WW or some other weight loss program if they feel they aren't getting what they need from SP or the community.
Of course, some people deleted their old spark pages to reappear under a different name. Some members were harassed by others. I know that stinks, but that has happened too.
Perhaps your friend was getting a little overwhelmed and needed time to think things out. Let's try to think of the positive reasons she might have decided to leave instead of the negatives ones. That should help you to feel a bit better.