Sara, you’ve asked a really good question which is quite complex.
I looked at your profile and noticed that you studied engineering. This is great, as it tells me that you value learning about the mechanics of how things function.
I wouldn’t want to suggest what might be effective for your client without having done a proper assessment. I will, however, suggest that you take the time to learn as much as you can about the shoulder girdle. As you do this you’ll have a better idea of how your clients moves.
Textbooks that I have found to be very helpful are Diagnosis and Treatment of Movement Impairment Syndromes by Shirley A. Sahrmann and Hollinshead’s Functional Anatomy of the Limbs and Back, written by David. B. Jenkins.
I sure hope this is helpful to you.