My best guess is that her pelvis has an anterior pelvic tilt and tight hip flexors with her low back in extension. This can sometimes cause the stomach to be stretched or taut. Or hard as you say. Then you do postural correction.
If it is muscle then, I highly doubt that.
Also may be bloated due to poor diet. Have them log the food and see if there is extra sugar or other major imbalances. Just some idea’s I hope this helps!