That is a hard situation to be in. One of DDs friends is like that. She is absolutely amazing on floor/beam. Perfectly tight body, straight lines, always has pointed toes, etc. She really struggles with bars. I don't really see them do anything different with her. I think that she will probably always struggle with bars. But hopefully it will just click for her eventually.
As for your DD, I would definitely talk to the coaches. Is there any way she could go to open gym and focus on her bars? I don't think I would pay for privates at this point, but maybe it would help her. It does sound like she really needs to focus on staying tight to do better on bars. I would talk to her and see what her thoughts are. Is she okay with scoring well on other events and maybe not doing well on bars? If so, I would let her compete and find a way to get her a little extra help.
Best of luck to her. It does suck for girls who are amazing on other events and struggle with one, especially if it keeps them from moving up a level. But sadly, gymnastics is a 4 event sport and it doesn't seem there are many options if you can't do all 4. For girls with really good floor, there is always tumbling, but many love beam and the rest as well. There is also rhythmic gymnastics, but I know in many areas it doesn't even exist competitively.