You don’t mention what the original reason was for the physician’s visit and what “your knees are shot” means. I would inquire further with the client as to what they were doing that caused the pain prior.
As Susan mentioned, Insanity is an intense workout and is tough on the knees as it requires a lot of plyo. The fact that he has no pain doing Insanity, but that a physician indicated he will eventually need a knee replacement is surprising and would cause me to ask, What changed in the last 2 years that the pain has subsided.
I love Karin’s exercise and its a go to in my arsenal of exercises for clients with knee issues. I also tend to work with in the clients range of motion. In other words, no exercises are necessarily banned, unless specifically mentioned by a physician. We just work within the range that is painfree for that client. With knee issues, I often work on strengthening the quad, glutes, adductors and abductors. Some knee pain for some people is generated from overly tight calfs and hamstrings. An assessment will tell you more, as well as how the person moves when performing a squat or walking.
Dr. Nick DeNubile has a wonderful book called “Framework”, that talks a lot about working within a patients pain free range of motion when healing an injury. Its well worth the read!