personally i have hyperextended knees.... i find it difficult to work.. but after a while... it has its benefits...
i would suggest that your daughter to do pilates. pilates helps with the abdominal strength and her core, and there are some exercises that involves the straightening of the knee (Leg circles). she could try this with a theraband (for resistance) when she is lying on her back, with one of her leg lifted, advise her to stretch the leg but thinking of reaching/pointing her heel towards the ceiling/sky, rather than hyperextending her leg which will result the heel to face her head.
and if your daughter does pointe, facing sideways to a mirror, getting her on pointe in parallel. ask her to hyperextend her knees. she could see how much 'curve' her legs are achieving, and from that hyperextension; let her relax her knees into a parallel plie, then straighten from her quads (imagery of her quads have strings attached to the ceiling like a puppet think of pulling upwards from the quads). she could eventually practice this with turnout legs, once she gets the feeling of pulling her legs up...
these exercises are some that worked for me... hope they will help you too