If the client is taking morphine for for backache, then there is no reason why this would contraindicate a microdermabrasion facial. Morphine is used for severe pain, more deep seated and intense pain and in terminal illnesses, so I would be more concerned as to why your client needs the Morphine ie. what is causing the backpain and will your client be comfortable lying on her back on the couch??
If you perform your sensitivity tests for facial treatments as per your training, then there should be no reason why your client should not have a microdermabrasion facial if she passes the test.
Morphine in itself would not contradict a facial. I would look more at the reason for taking the morphine.