I have seen this sort of person on forums. For some reason, the internet seems to bring it out. It isn't always sexual abuse, sometimes it is something like a miscarriage or their having depression. But whatever it is, the person will bring it up all the time, in completely inappropriate threads. One of those worst examples of this was a thread about early childhood memories..it was really lighthearted and fun, people were posting about their memories of preschool...and then one person came in with "I remember being assaulted by a family member. That destroyed my trust in humanity." or something along those lines...well, really, how can you go back to posting about finger painting after that?
If I were in this situation, I would address it in response to what she says, rather than taking her aside to have a talk. For example, when she posts insensitively to the woman who recently lost her father...I think that's a good time where someone could say 'Hey, that was insensitive! Everyone has had bad things happen to them, and turning it into a contest to see who's had it worse is really unfair." or something. And when she starts a thread about how everyone's ignoring or being mean to her? That's the time to perhaps send her a private message. She has basically brought the issue out, so saying something like "hey, I'm sorry you're feeling ignored, but here's the way I'm feeling, and I wouldn't be surprised if others felt the same.." isn't uncalled for.
I feel it would be more of a shock and seem like a ganging up if someone did it without provocation at the time, whereas addressing each post as it happens seems more organic.