I am assuming you mean reverse shoulder surgery. It will take about 6-8 weeks for her/him to be back to training after physical therapy. Given the doctors recommendation, proceed slowly with traditional shoulder strengthening exercises.
My client 76, has been through that surgery after tearing 2 rotator cuff tendons in a fall last year and the initial shoulder surgery was unsuccessful.
In her case, I also found that she was in so much pain, that she almost did not move at all. Her posture deteriorated and she became overall very weak. We work on her core, legs and stamina in addition to some shoulder exercises mostly through Pilates.
Quite frankly, I’m not sure what you mean by “reverse shoulder replacement” surgery. It’s laudable that she wants to do some preparatory strengthening, but I definitely agree with Sue that you should consult with her PT and/or surgeon to determine the best resistance exercises for her. Shoulder abduction, horizontal adduction & abduction, flexion and extension, perhaps internal and external rotation, could all potentially be part of the mix. Just clear with her providers prior to beginning a program. I wish her the best.