Knee injuries can really put a damper on the enjoyment of cycling. When we started riding a tandem Mrs. Dan and I had very different cadences.
I have had several knee surgeries, and after the last one was told by the surgeon who repaired my knee that he saw very little wear damage for a long term cyclist and that he believed it was probably due to keeping a faster cadence. I try to keep my cadence right around 90, but can go down as low as 75 for a short time. If I push too hard at too low a pace I have a sore knee.
Mrs. Dan on the other hand was formerly a 60-70 cadence rider. At about 60 years of age, we started out riding 80-90. After quite a few years Mrs. Dan is now quite comfortable at 90 and can go a lot faster for a short time. So, yes, people can develop a faster cadence. And riding a tandem has allowed us to ride together with two very different levels of strength.
I know that this is going to be a dirty word to some people, but has he considered an electrically assisted bicycle? We met a couple when we were touring the Isle of Skye (a fairly hilly route) and one of them was dealing with Ankylosing Spondylitis. She had a bike with an assist and they told us that it had given her back that part of her life.