I wouldn't feed my baby Pediasure. Not with the loads of sugar it has in it. You want a baby to gain weight, there are FAR healthier ways to do it than stuffing them with Pediasure. Avocado and bananas, for one thing. Which is what I did when my oldest son's Pediatrician told me he thought we should do something to put some weight on him. He was always in the 20th percentile, if not lower. He steadily gained weight, but he was always quite small (weight wise). Instead of loading him up with sugary things like Pediasure, we increased his HEALTHY fatty food intake. He gained more weight, but he also evened out. He never became a "chunky" baby, and at 12 years old he is a very healthy, lean weight. Despite his family predisposition to be morbidly obese.
If her baby is eating solids, I'd suggest getting him to eat more bananas and introducing him to avocado. It's far better than the high sugar content of Pediasure.