Rear Delt Rehabilitation

A friend of mine was invloved in a motorcycle accident about a year and a half ago, and was thrown shoulder first into a tree at 50 mph. The impact basically smashed all the bones and muscles in the vicinity. Most of the muscles have since recovered almost completely, but it seems that the posterior portion of the left deltoid fails to respond to any training. This also limits the weight and range of motion in many other exercises. I have heard that EMS can be useful in situations like this. Any ideas for rehabilitating this shoulder injury would be greatly appreciated.