I strongly believe you should keep your nose out of it. You sound very naive, and almost nothing you could do here -- especially because you're leaving -- could help. Almost anything you could do would make things worse. It would look like you leaving a bag of crap on the porch on your way out of town. If your boss has a problem, he needs to bring it directly to Anna and not use you as an informant. (I also wonder if you're quite young, but I don't want to presume on the internet.)

The reality of life is that many people don't like their jobs (especially "unrewarding" ones), but have to keep them. That is not a crime! That's life and we all have to make a living. In this economy, no one can afford a "kick in the pants" to find a new one. This person may well be unpleasant, but think seriously about whether it's ethical to jeopardize a person's livelihood for having a"bad attitude."

Now, if she were embezzling, abusing patients, etc., that would be more serious. Making you uncomfortable is nowhere near that league.

That she would complain about you when you're not present -- why are you so surprised? What's more, why is that so terrible? You're complaining about her on the internet. Another reality of life you seem new to: Coworkers, even ones who otherwise get along well, do bitch about each other sometimes or even frequently. It's called venting -- the office environment forces togetherness under sometimes-stressful situations. Now, most people have enough discretion to do their complaining outside of the office, to trustworthy non-work friends and family.

None of this is to disagree with the fact that your coworker sounds very unpleasant to work with. I'm sure that's true. But don't consider this fact to be some kind of troubling scandal or a problem where your "help" or input is warranted. Just consider it an opportunity to learn about human nature.