The doctors told him that pretty much anyone who jogged that much has to get new knees.
Running is a complex biomechanical activity. Most people I see running are not running with biomechanically-correct form. This probably stems from lack of knowledge of how to run correctly, lack of core strength to run correctly, shoes that do not fit their physiology and personal running form, etc., etc.
Since most people run with poor form, it's not a surprise that most people that jog require knee replacements.
Running, when done correctly, produces minimal stress on knee joints, even at 10+ mph.
Modern padded running shoes promote bad form, causing knee and other injuries, and prevent your feet from strengthening, causing planar fascitis and a few other maladies. Your foot is actually well constructed to run, but it can't do it's job wrapped in a ton of leather and foam.
I've had some success with minimalist running shoes (abrasion protection only, no padding, sole is about 1/8" thick)- it's important to enable your feet to strengthen. After a few weeks of walking around in thin shoes, I started running again and it felt like I had new feet- it was awesome.
Wearing thin shoes forces you to land on your forefoot, allowing your very complicated foot to absorb shock like it's supposed to. Wearing thick-soled shoes allows you to land on your heel, and that force is transmitted straight up to your knee. The padding prevents immediate pain but the shock goes through nonetheless.
There's a great book on running, called "Born to Run", that discusses this and many other aspects of running. I highly recommend the book.