Speaking as a mum of 2 'squad' gymnasts, one with HMS who has had some difficulties with joints etc:
Main thing, as you have already noted is that, as it stands she is just bendy, no problems so far and quite possibly never any HMS related issues - so a great pity to stop her because of something that might never happen.
That said, the main potential problem with HMS and gymnastics is that the mucles lack the strength and 'knowledge' or 'muscle training' to support the joint in it's abnormal range, resulting in a relatively unstable joint. However, gymnastics has 2 main elements, 1) learning all the moves and skills 2) strength and conditioning training. Your coach could possibly be advised that she MAY suffer from unstable joints and that special attention should be paid to strength and conditioning (ie she may need to do more than the others). She may benefit from extra work on this at home.
My personal thought are that the work that my daughter does in the gyym, while it occasionally results in injury actually means she is able to function much better in daily life that she would otherwise have been able.
