So what should we do?
The obvious first step is to try to avoid the negativity. We all have people in our lives who have certain topics that they continuously gripe about, so try to avoid those topics. If a Negative Nelly starts in on her favorite grumpy topic, direct the conversation to more neutral subjects or things that the person enjoys.
If that doesn't work and you truly care about the person, you can try putting yourself in her shoes; if her complaints are justified, express empathy and maybe make a few suggestions on how she can improve her situation. Keep in mind, though, that Debbie Downers probably won't want to hear your opinion. They are too far entrenched in their victim mentality and have lost all self-awareness and don't want to entertain any views other than their own. They just want an audience. Even worse, they often don't take an interest in you---it's a completely one-sided relationship in which they drone on and on and on about all of the rotten things that the world inflicted on them, and never have time for you unless you're complaining as well.
If you want to salvage the relationship, you can try to be blunt and point out the constant negativity. Let the person know (nicely) that you are very aware of her complaints and you have nothing new to add to the topic and would like to discuss something else.
If that doesn't work, you simply have to limit all contact. Why spend your valuable time with someone so toxic?
I know everyone goes through rough times and needs to vent. That's normal. And some situations never change, so the complaints are probably well-founded. But if that is ALL someone focuses on, it can be very draining and a huge waste of time. Life is way too short!!