When I adopted Sierra she was underweight and stayed that way for a few years. I free fed her for almost 2 years, but never gained weight.
It wasn't until she was 4 or 5 years old that she started putting weight on.
My vet said she was healthy and there wasn't anything to worry about, but I was SO tired of hearing "Is your dog sick? Why is she so skinny?"
Grrrr.. that peeved me off to no end.

Your friend could try free feeding for a while and/or add high quality puppy food to her diet.