My friend has a few mental ilnesses and her being depressed is making me depressed. What should I do?

We've been best friends for ages but a few months ago she was feeling depressed and was diagnosed bi-polar and a few other conditions and started seeing a psychiatrist. She's been really depressed and isn't eating very well anymore which has made her really thin. Seeing her like this is making me depressed and I feel like I can't do much for her apart from comforting her. Today, she's been in a really snappy and irritable mood and I didn't think much of it since she has bi-polar so she can get in really bad moods but later on in the day she went in a mood with me and said she was annoyed with me, when I asked her what I'd done, she slapped me and said, "Lots of things." Then she stormed off. Tonight, after school, I spoke to one of her close friends who was really upset and when I asked what was wrong, he told me his cousin died yesterday and said he was shocked that my best friend hadn't told me because they were really really close. I wonder why she didn't tell me... now I think about it, even when I asked her how her weekend had been she said, "Yeah, it was alright.". I just don't know what to do. I really don't want to lose her as a friend but she's changed so much over the past year and I just don't know what to do. Any ideas?