Actually happened to a friend of mine, about a year ago.
The girl he was messing with had cancer. He knew before, though. She only had months to live.
She wanted to get really close to him. He cared about her too, but he didn't want to get too attached because he didn't feel comfortable knowing she'd be gone soon.
A few months before she passed, he stepped back from her.
She was crushed (from what I've heard).
When she passed, a few of her friends villified him and called him all sorts of names.
Crazy stuff, man.