My Daughter Found This Prescription For Diclofenac Sodium Topical Gel

My daughter found this prescription for Diclofenac Sodium Topical Gel aftger we moved. I lost the little plastic stick that came with it when I opened the package (the med has not been opened). My husband has a bad knee that gives him pain. He just saw his osteopatick surgeon and there is nothing wrong, so it is arthritis. We live in an assisted living home and they have been using BioFreeze on the knee whenever it acts up. Also with a Narco pill. Can he use this Gel and how much should he use?