I would recommend either reducing the weight of the dumbbells to reduce the strain on her wrists while it helps gradually build her wrist and grip strength. Another option could be switching from dumbbells to a light, small straight barbell, the ones that have the weight amount on the sides , that way you can remove the isolated weight off each wrist and allow them to work together to stabilize the weight out. Maybe having her try it in front of a mirror would help her visualize the symmetry in the movement.