Is assessment and corrective exercise within the scope of your practice? You might need to refer your client to a doctor or physical therapist to see what muscles are involved with her pain, and what’s safe to work and not work.
You might be able to do some simple assessments to determine which actions are possible without pain. For example, since the tricep involves two joint actions, maybe you can eliminate one and reduce discomfort. Instead of doing an overhead press from standing, what about attaching a band or tubing high and doing the tricep exercise as a pull-down – good range of motion and doesn’t put the shoulder in an unusual position.
I have one client who is recovering from a rotator cuff tear (has seen doctor and cleared for personal training) and with her I use lightweight tubing that’s attached to a pole at the height of her elbow. From there, she faces the pole and does elbow extension. It’s a very small range of motion and a very light amount of tension, but I can get her to locate (“feel”) her tricep and isolate it from the other things happening around the shoulder.