I have a friend like this. He goes in and out of these real self-deprecating periods where he's the scum of the earth, nobody loves him, etc etc, and also that life sucks, there's no point in living, etc etc. And he's a grown man, around 40 or in his early 40s. I've known him around 10 years and he's been like that the entire time I've known him. I have actually had to tell him that if his attitude didn't change I would have to discontinue our friendship. He does suffer from hardcore depression and a history of severe childhood abuse, but at the same time it's impossible to have relationships with people like that. He would always tell me - still does - that I'm the only person who understands him, the only person who he loves or holds dear in this world. But I have still had to tell him, Look, it's impossible to be around you. You're negative all the time and by saying the things you do you're not only disrespecting yourself but the people you care about. He straightens up (for awhile at least) whenever I've said that to him. For me, it's not a primary enough relationship for me to have to choose one way or another whether to keep him in my life or let him go with any sense of urgency, so I've been able to keep him in my life.
There are ways to deal with people like this. I know, because I used to be one of these people. When she calls and starts complaining, just let her know that you either don't want to talk about this again or that you don't have the time. If she refuses to talk about anything else or gets angry that you don't want to listen to her complaining, you might want to have the 'Look, this is how it is' talk with her, depending on how good of a friend she is. If she's more of an acquaintance, you might just want to decrease contact with her or end it completely.