I want to tell a friend she needs to stop hanging out with us
Why do you want her to stop hanging out with you and your friends
She used up to find her current boyfriend.
It's understandable that you no longer want to be her friend after she used you to get a boyfriend. Staying in a friendship with someone who uses you will only make you feel unworthy in the long run. You should look for people that want to be friends with you for your personality.
Nah, I'm going to tell her I don't like her anymore, so that I feel better about it.
You are entitled to do that. She might have other motives so please try to listen to her when she replies. You might be able to save your friendship.