My vet always recommends some canned pumpkin (1/4 tsp) mixed into the cat food when they become constipated. But I don't think this would be good as a long term solution.

Your friend can't make the cats play? Or if she feeds them food, try putting the food on top of higher objects so the cats will have to climb to reach their food. If she has a staircase, she can start feeding them upstairs so the cats will have to climb the stairs to get to the food.

I would really try to make an effort to excercise the cats daily.