What is my friend's problem?

last year three of my friends and i formed a flute quartet. this year, one of our directors pulled one of the girls out. now, with us three remaining girls, the director told us to do a trio and picked out a really nice piece for us to play...the only problem is, one of the girls left is being really childish. she refuses to work unless the girl that left comes back, and she's really clingy towards her. she only wants to be with that one girl and it's getting in the way of our practice time...we have only 2 months until competition and we haven't put anything together because she'll throw a fit if we have a rehersal without her 'dear nicki'. guys, what is her problem?? and, should i kick her out or try to talk to her first?