Personally, apart from - as you mention - trying to convince her to get some professional assistance I'd stay well out, Tina. If she truly is irrational, heaven knows what affect any well meant advice may have on her. If you do attempt to convince her to get some help I think I'd leave the broken relationship out of any coaxing and simply concentrate on the fact that she is under stress at this time ... Then again, I'm no psychologist!
[cross-posted with Duj] How very difficult for you. Are you in the UK? Can you point her towards the Samaritans or even Relate? In my experience [I've been a voluntary counsellor for some years] you are doing the right thing, in that you are listening. If you are happy to be just a sounding-board, for that is all you are, try not be too disappointed if she fails to heed your warnings. By confiding in you, I'm afraid it looks as though she is seeking justification and perhaps your approval for her future actions, ie. she is likely to meet up with him. I would say, your main problem is that her irrationality appears to have transmuted into outrageously selfish and controlling behaviour [towards you] and you have to guage how much more you can cope with. When she's next in 'listening' mode, you might gently point out that you have a life aswell. But unless she admits she needs professional help, there is little you can do, short of frog-marching her into a doctor's surgery. [Blimey! Do I sound like an agony aunt or what?]
Yes, this is what my sister says as well. I'm in the US and our employer has a counseling plan. My sister says, make excues when she comes to my cubicle. Yes, she is always seeking my approval. She will announce some awful plan, come to my desk, and ask what I think. I say I disapprove, and so far that works. But it's not right really. I'm not the professional. And she is selfish now. There is nothing in her life except her crisis. She lost maybe 20 pounds. She writes me at 3 a.m. and is not sleeping. Thanks for all your help. I just don't want to open the paper and find her dead.
I cannot put myself in your position and therefore cannot really understand, but it sounds like you are in a most awkward position - particularly if you feel obligated to your co-worker. Chalky is undoubtedly correct when she mentions listening but this, long term, could interfere with your own work. In view of the fact that your employer has a counseling service, why don't you have a talk with their staff? Depending on legal contingencies you may have to keep the name of your fellow worker out of any discussion, but at least you may receive 'proper' advice (and by that please do not think that I am denigrating Chalky's input.)
Tina] If you are concerned for her life then she definitely needs professional help. I think you have demonstrated the characteristics of a good listener - but you are not responsible for her situation. She has to be, so it may help to explain this to her in a kindly and assertive way. She may thank you for it.
I would echo what Dujon said in his last post - counsellors can be extremely useful even if you aren't the one who is in direct need of their services. It allows one to perspectivise the situation. Obviously the best case scenario is if somehow your colleague manages to sort her situation out; the worst case scenario is if this doesn't happen and you get dragged down with it. I found a counsellor was enormously helpful in staving off the latter when I was in a tricky situation.