I'm going to caveat this answer by informing you I am not a physical therapist. The routine I'm linking to is from the American Academy of Orthopedic Surgeons: Rotator Cuff Shoulder Conditioning Program. Some high points are:
- The rotator cuff needs to heal. If your client has pain dealing with any of the exercises outlined, don't do them.
- Strengthening all the muscles surrounding the rotator cuff will help the shoulder stability and prevent the rotator cuff from having to bear the weight it bore when it tore.
- Keep any resistance light.
- There's a lot more movement patterns than external rotation needed to rehabilitate the rotator cuff--though external rotation is certainly in the selection.
You really do need to interact with your client's doctor (with your client's permission) to determine the extent of the damage, and the doctor's recommendations. How soon the client can resume exercise and the type of exercise really depends on how bad the tear is. If it's a minor tear, then rehabilitation can start pretty soon. However, if it's a major tear you may need to just rest it for a while.
Another article that may be useful discusses whether physical therapy can prevent the need for surgery in bad cases. NOTE: in many cases it can (even full thickness tears). Good rehabilitation requires the following:
- Restore shoulder mobility
- Restore shoulder stability
- Reduce the impact of the kinetic chain (i.e. correct any impingement or dysfunction in the shoulder).