Yes, she has the right to live her life as she pleases.... and you have the right to choose whether or not you get involved with her.
If what she is doing doesn't feel right to you, then it's not right FOR you.
If her relationship with the other one isn't serious, then there would be no expectation of exclusivity, and therefore there would be no problem hanging out together. If it is serious, and she is truly poly, then there should be no problem hanging out together. Any other outcome would be a nice array of prettily-coloured flags for me, mostly in tones of red and yellow...
"Listen, or your tongue will make you deaf." - Native American Proverb