Do I move on or still tell her?

Here's what you should do. Don't tell her anything about how you feel about her -- that's the worst idea ever! But you're also not going to completely forget about her either. Keep letting her talk to you and approaching you, and be ready if she ever severs ties with that guy that she is in an "open relationship" with. In the meantime, hang out with your buddies and date some other girls -- it'll get your mind off of this one!