During the last year and a half, the recovery / rehab from knee reconstruction in April of 2008 (replaced ACL, mended MCL, miniscus trimming, resurfacing of patella and femur - the full meal dead, and by many accounts a much more difficult recovery than replacement) is what led me back to serious cycling from a 30 year hiatus.
Although before the surgery I had been riding a cruiser around town and spinning at the gym, as soon as I was able after the surgery, I got very serious about spinning. Then this spring, my wife and I took the cruisers to do the 35 mile section of a local club ride, and a few months later, did a 50. It was at that point that I was bit by the bug. Later this summer I got a serious road bike. My wife who commutes to work on her crusier was inspired to get a road bike as well.
Over the course of the summer, I have progressed from thinking at the first club ride, "This is fun, but those guys in spandex riding a hundred miles are nuts," to yesterday, completing my first century myself. My wife did her first metric century.
Because of the miniscus wear and questionable ACL in my non-surgical knee, I don't run or play tennis or vollyball. I can ride and plan to do so as long as there isn't ice on the road. When I can't ride the road bike outdoors, I'll continue to do spin classes and ride on the trainer at home. I feel as though cycling has given me a new lease on my athletic life.
The point of my story is that your friend's knee replacement shouldn't be a reason not to ride, it shoud be a reason to ride.