I have a client with a double hip replacement, and Nancy is correct about the age of the replacements. Mine should not cross over her center line or perform exercises that externally rotate/expand her hips beyond hip distance (think plié squat). Her replacements are about 20 years old. If your potential client becomes a client, I would consult with her doctor to get any limitations.
I’ve also had clients with a double knee replacement–same concept about consulting with her doctor, but I’ve not had any limitations there (each client is different).
Best to you,