Maybe if you can suggest she go see a counsellor so she has someone to vent to that can also give her advice. I know for me it really helped when I started talking to my doctor, and joined a mamas group. Finding people who could relate and understand how I felt really helped me deal with a lot of the changes happening in my body.

I would also recommend encouraging to get out with her friends and family to talk. Getting out of the house would probably be good for her and would allow her to rant without you having to hear every irrational thing she is saying. If this all started from the pregnancy she may not even realize how bad or hurtful she is being. I am pretty self aware and even I had a hard time controlling first trimester hormones.