How does her difference in "values and priorities" effect your life? How does her being at the same social functions make you have a less than fun time? Are you actually giving up your power to her? Do you have a difficult time creating boundaries, saying no, or communicating openly, clearly, and directly for fear of "hurting someones feelings" or "not having someone like you"?
When you ask the question "Why is she doing this?", what is the answer you automatically have in your head? Do you believe she is doing this to you on purpose? Do you believe that she is trying to steal your friends away? Do you have the judgment that she shouldn't be doing this? Do you have any way to know this is really true? One of the dangers of the stories we tell ourselves in our heads is that usually they are not even true. This means that we walk around feeling a particular way based on a made up story.
She can only make you uncomfortable to the extent that you allow her to. I know this sounds like a pat answer and yet, it is true. We often believe that any discomfort we feel is due to how another person behaves. Actually we feel uncomfortable because of a belief we have about their behavior. Usually that belief says they "shouldn't" be behaving that way. Well, wether they should or shouldn't, they are. Now what choices do you have? You can speak to her, you can let it go, you can choose to not attend, you could choose to understand why she is really behaving the way she is (maybe she is really lonely or afraid), you can look at yourself and find what is inside of you that is getting triggered by her.
No matter what her behavior is, you are the one that gets to decide what your response will be. This is what maturity is all about. We stop blaming others and the events around us and take 100% responsibility for our own emotions and behaviors. We learn to evaluate what we are believing, how we are judging (saying they should/shouldn't be that way), and choose differently. Clearly trying to change her or even the circumstances isn't going to work. Not fair? Yes. And you get to choose whether you will let other people keep you imprisoned or whether you will step up and be empowered by making choices for yourself regardless of how others behave.