Is it muscular distrophy?

My friend has a medical condition that I'm curious about, but I don't want to bring it up with him. His arms and legs are extremely thin, lacking most/all of their muscle mass. The arms are "normal" from a little less than halfway down the radius and ulna and up. However, starting about 4 inches below the elbow, the muscle mass steeply slopes off to the point where you can see the space between those two bones. He has most of the function of one hand, but the other is paralyzed.

His legs are not much thicker than his tibia (at least from the knee down), but he is able to walk fairly well.

Does anyone know what this might be? He's scrawny, but if it weren't for the arms, you'd never know there was anything wrong with him. He doesn't have any scars that we bely a traumatic accident as the source.