I believe you're not the only one who noticed that coworker of yours is like that, especially if she said those rude things while being with other people just for the sake of criticizing the absent person.

However, people usually don't do anything about that, simply because you can't change someone and in order not to have a confrontation with that person. I had this classmate of mine who used to do the same. She was friendly with everybody but when someone wasn't present she loved saying bad things about them. And I wasn't the only one who noticed. At first I thought so, until one day someone else mentioned it. But what else could I do? If I told the criticized person what was said about them I'd get in trouble, and no matter how many times I could try it, that classmate of mine wouldn't change her attitude just because I told her to. I also had the feeling she hated me, and I've always believed it was because I knew how she truly was so I didn't talk to her much.

In my experience, it's better not to pay attention to that person. It's completely normal to believe someone is actually mean if there are signs that make you believe so. If she says something similar in front of you just ignore her. And don't worry about what she may be saying about you (if you ever do), it's not worth it. What she says about others says more about her than about them.