Oh Jill, I am so, so sorry. I have many family members with a variety of mental illnesses, and depression ranks near the top of that list. My brother has it especially badly and it is enough to tear my heart to pieces sometimes.
Here's the bottom line: You are not her doctor. You are not her therapist. You are her friend, and that entails a lot of responsibility - but this is way beyond the scope of your "duties" as a friend, or even as a family member (or for ANYONE who is not her doctor or therapist!). Furthermore, her constant reaching out to you is a way of avoiding her personal responsibility, which is to take care of herself, whatever that means. It might mean an in-patient facility (I think this might be the wisest move). No matter what the (financial) cost, this would be worth it, if she found the right place.
It is very difficult to give tough love to somebody who's depressed, or on the edge, or already very tough on herself. You don't want to add more shame to their self-loathing, so you do have to be tactful, but you also must be firm. For the sake of this discussion I'll refer to her as "Jane." I would say something like:
"Jane, I am so sorry you are going through such a wretched time, and my heart breaks for you. I wish so much I could take this pain away from you. I am terrified for you and you need to get help. I'm not judging you; I love you and I want you to get better. But I cannot make you better. I promise your thoughts and feelings are not rational, nor are they an accurate reflection of reality. You don't sound at all like yourself. I think it's time you (saw somebody/checked yourself in somewhere/etc). I'm not in a position to help you, as badly as I wish I could. You need to get professional help. Please let me know (who/where/etc) I can call to get you treatment."
Something along those lines. This is a major breach of boundaries and you need to protect yourself and your family at all costs.
Hugs, I am so, so sorry. I know how painful this is, it's gut-wrenching, and the heartbreak and helplessness felt by friends and family members of the person with depression is almost too much to bear.
ETA: I mentioned an inpatient facility because she would have the time to try and tweak different medications until she found just the right one before she left to be in the "real world" again.