Any suggestions on how to get a dog to GAIN weight?

My friend has a boxer, she's about 2 years old. She is pretty small, at least I think so. But she is so skinny her ribs are showing. Her vet says she is healthy and doesn't seem concerned, but my friend always gets comments from people about how skinny she is. Me being one of those people. Honestly, I think she is too skinny.
Here is a pic of her from about a year ago.

What do you think?