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!
If her stomach is distended and hard, chances are good that she has too much visceral fat, (that is the fat that develops and forms around internal organs usually either from a sedentary lifestyle or chronic stress). This fat is different from the fat that we typically see in clients who have “soft” stomachs. Although both fats are not good in excess, visceral fat is especially not healthy. Great way to reduce both if she has been cleared by her doctor for exercise, is to do HIIT type training, cardio, and a good diet. Hope that helps!