Some time ago I had a rotator cuff injury that caused me to change my lifting mechanics without even noticing, my body found it's own way to work around the pain. For example when I was doing curls I would use my trap on the injured side to elevate the shoulder and take away some of the torque from the rotator cuff. Like I said I wasn't even aware of it until one day I noticed that my traps were seriously uneven, my biceps' were a little uneven too but less noriceable. Sorry for the long story, but this might be the case with your friend too.