I have been this friend for the last three years while managing DH's mental illness. My friends support me in different ways. Do one or more of these things:
1. Keep listening and don't judge. Read up on mental illness so that you can discuss options with her.
2. Relay your own problems to her so she knows your life isn't perfect.
3. Send a grocery gift card. It's such a relief to know you can get groceries without cutting out something else.
4. Tell her not to be ashamed of her husband's depression. I told a bunch of people so that they would know why I was acting weird.
5. Send her a book or a magazine subscription if she likes to read. She can lose herself in that when things get stressful.
You're a good friend!