Well - coming from a family with a lot of depression but with only minor symptoms myself I can only add a few stray points:

- exercise does help. And "forcing" her in a friendly way might be necessary.
- being able to see ones depression from the outside is a good thing! but doesn't lessen it much, i.e. things like being unusually touchy when not invited to things can hurt just as much even if you know it's irrational.
- Depression can make you incredibly self-centered, and not always very nice. Realizing this isn't much fun either.
- having a place where you HAVE to function and CAN'T feel like a victim can help. Whether it's work, being a parent, or volunteering. Helps self-esteem, helps lift your thoughts, gives new input.
- You as a friend cannot cure her depression. You can help her in a few ways, suggest activities and go with her, but you cannot assume responsibility for her well-being.

Just some random thoughts. I'm sure people here who have been through "the real thing" will be able to add more. Best of luck.