It might be pain related teeth ect. The owner must get her horse vet check. The horse is telling there is something wrong. Not natural if a horse does not want to bend at all to the one side. Maybe a good idea to be vet checked. If the horse's teeth have been done recently it might also be tender on that side of the mouth and they will try to avoid to turn because the bit is hurting. If all is good then it then maybe more ground work to get supple. Lots of bending, step under ect. Hope it helps.
Supple her by using lots of leg with a supportive hand that encourages her to know you want to turn left/right. Lots of circle work: if you are cantering down the long side then ask for a flexion doesn’t matter if you chop and change, keep her mind on the job of “ok we might be turning now” so go straight then do a 10m circle and repeat a few times just don’t always be going straight or doing the same thing. What I have grown up with is inside leg to outside hand using the inside rein to ask for the turn and determine how small you want it.
You can always ask your trainer for tips since they might know the horse well, or ask whoever does know the horse. If it’s not medical, work on circles a lot and randomly trot around. do figure eights, make random 10, 15, or 20 meter circles whenever you like, and always mix it up so she has to be responsive and listen to you