Get clearance first, then try having the client move the rest of their body while keeping the affected arm in on place. Slow and controlled movements. Move their body in ways that make the arm move a little in all directions. You could have them hold a stationary object like a door knob or a handle in a fixed place. Then move the body with the arm straignt to almost straight, not locked out at the elbow though. Keep the reps low and the discomfort very low. I can’t really suggest more than that without seeing the client personally. Make sure their doctor is onboard with this first. Unless there is a serious structural issue, this should not be a problem. If it begins to open up the ROM, proceed slowly. Most people get over encouraged when the motion starts to improve and overdo the exercises.