Of course the obvious answer is… ask their doctor or physical therapist.
Here’s my two cents from personal experience though- my sister had carpal tunnel from working at a computer all day for the last 10+ years and I have really weak wrists from being a dental hygienist for the last 6. We both opened a fitness studio last year and we were the only 2 instructors for a long time. She was teaching SURFSET (TONS of planks & push-ups) and was in a lot of discomfort for a while. She even thought about wrist guards. After teaching for a few months and doing the exercises little by little with modifications, she doesn’t complain of pain at all anymore. She thinks that her wrists got stronger from it. Mine also feel much better since I have been doing PiYo.
When I did do personal training for my sister, I just listened to her and modified if something hurt. A few modifications I did were:
*light resistance bands for biceps curls instead of dumbbells
*tricep push-ups on her knees
AGAIN… I’m not a doctor, this is just my personal experience with it.
Hope this helps 🙂