before going to the coach, talk to your flyer. if she is being loose or lazy, tell her it is difficult to do your part when she is flopping around up there. ask her if maybe her foot is uncomfortable, or if she feels off balance. there may be a reason for her looseness. if she just isn't trying, like she pulls a stretch lazily, or never stretches her splits so her flexibility is atrocious then tell her she needs to stretch. make a comment on how gorgeous the point flyers flexibility is, and then tell her that she is going to look like that.
however, if she is just being a brat about it and refuses to cooperate, then speak to the coach. explain that you want this group to work but your flyer just doesn't seem into it. say that you spoke to your flyer but you didn't make much of an impact. and then have your coach watch your stunt group each practice so he/she can add corrections and criticisms.