First, tell the forum friend "are you ok? if you're feeling bad then you can talk to me" or something like that. Then tell your IRL friend that this behavior isn't OK and that she should stop. Tell her to apologize to the forum friend.
Maybe you could try re-introducing your IRL friend to the forum friend as a nicer person, and the three of you could all be friends!
Try not to tell the IRL friend online to stop, as this may result in a bigger flame war than before.