This is not you or your girlfriend's fault, and your lass needs to realise this. If Lucy can't keep herself under control, is that supposed to mean that two other people can't be happy?
The girl is probably using this as blackmail, or it is a strong initial reaction to upsetting news. As has been said, she may well calm down.
What your girlfriend needs to realise is that, upsetting at it might be, what Lucy wants to do is Lucy's business and Lucy's responsibility. Yes, they went through a lot together but just because someone has been hurt, doesn't automaticly make them sweet, innocent and a victim. Lucy is not acting like the supportive friend that she should be and your gf has absolutely no responsibility to deal with that.
Why does your girlfriend care more about someone she has not seen for months than she does about her boyfriend? This seems to be a guilt thing so you need to show her that Lucy is the one who has responsibility here and that her actions are complete over reactions. Is your girl just going to keep sacraficing for her? If Lucy can't get along and play nicely, that is HER problem.
What would she rather, lose her boyfriend and have to put up with an emotionaly unstable and manipulative semi-friend, or lose her friend (possibly by handing her over to a shrink) and keep her boyfriend?