not a physiotherapist but a retired occupational therapist, however I have worked with physios and have got knee issues (cartilege) myself. This is a "piece of string" question.
The answer is it depends on what the state of your Dad's knee is. Has he had scans, x rays, etcet and how does he know its arthritis? Is it just the one knee or is it in other joints? Who suggested the physio?
If he doesn't want to have pain then I am afraid some painkillers are unavoidable. I agree with Sqad about vitamins and substances that are supposed to "lubricate" or "resurface" the joint. Much wishful thinking and placebo and little objective evidence.
What the physio can do is give exercises to do daily or more often that will strengthen the muscles that support and stabilise the joint if this is possible. They can advise on a suitable walking aid, or the Occupational Therapist can. An OT can also advise on the best way to get through everyday activities and can suggest (and, depending on your area, may provide) equipment to help as well.
Any further questions, do shout :)