I'm losing my so-called friend...?

I have a worst enemy, let's call her Betty. Anyway my friend (let's call her Ellie) always said she hates Betty too but now she's been hanging out with her loads. Me and my other friend got really annoyed so we asked her why she said she didn't like Betty when she clearly does. Ellie said, "Um I like Betty why do you think I don't?" and we said, "Um because you said you don't! Why did you tell us that? You can be friends with her but we don't like you lying to us." and she said, "Because...because...oh never mind!" and she got all stroppy and ran off with Betty. Now Ellie's been avoiding us ever since and she's been hanging out with Betty. Then a few days ago at school in ICT she sat with me and my other friends acting like nothing had happened. Then Betty came in the room and Ellie told her to sit next to me and my friends (so that she could sit near her) so me and my friends just got up and left. You probably think we're being childish but we can NEVER forgive Betty for the mean thing she did to us so don't tell me to forgive and forget. Anyway shall I ditch Ellie? Or shall I give her another chance?