Feeling defeated about failure to thrive

My friend S just found out that her 2 1/2-year-old is being officially diagnosed as Failure To Thrive. He is a happy, connected kid who is hitting all developmental milestones, and is ahead of things verbally (in two languages). His caregivers at daycare give him glowing reports and he's just generally a funny, engaging kid. But he's not gaining weight.

For years she and her husband have been trying to get extra calories into him, but he's still not gaining weight the way her doctor wants him to. He's always been on a low curve percentile-wise but is falling off that curve, and his head circumference isn't growing like it should, and his height is lagging, too.

I recommended a) a second opinion, and b) finding out if he has food allergies or sensitivities that could be preventing him from gaining weight.

I know some of you have dealt with this and would love both some suggestions of things for S to look at, but mostly some encouragement. She's feeling completely defeated and inadequate as a mother at this point, and needs some help.