I actually have an online friend that we believe this happened to. He doesn't talk about it and none of us are going to broach the topic...but....
He was divorced after a ~25 year marriage, and he fell in love with a girl on the internet. They wrote each other poems and exchanged tons of emails, and I believe they even spoke on the phone. It lasted for a couple of years, if I recall. They never met in person. One day, she posted something super vague about being ill or something. And not too long after, there was more vague posting about how she wouldn't be around much because of treatments. We surmised that she had cancer, though she never said exactly what was wrong. And then she couldn't even communicate with him. Supposedly she died at some point. Maybe. We don't know. He continued to post poems to her every day for several years after she stopped communicating with him.
There is no doubt in my mind that he was really, genuinely in love with her. And we're pretty certain that she was a real person, but I think she was stringing him along and thought of it as a fun game, and then it wasn't fun anymore so she faked her death.
I believe this could happen to anybody. When feelings get involved, things get murky and tricky and "I would never do that!" kind of gets forgotten... I don't think this is any different from people you've met in the flesh who trick you, play games with you, or turn out to not be who you thought they were.