I just saw your pics andrw, good job!
just a question, what kind of workout are you doing? My friend is a bit overweight and is trying to burn fat but his knees hurt from running, sometimes from walking too. Is there anything else besides running, walking, swimming etc..?
I've been on Primal Blueprint/Paleo dieting since January-ish. As far as my workouts go, when I go to the gym now, I don't worry too much about cardio when I get in the gym as much as I focus on strength training. I'm doing 3-5 hours of low impact cardio outside of the gym a week now. You might have him look into doing it that way as low impact in and of itself is much easier even over that amount of time vs. 1-2 hours of high intensity cardio.
When I go to the gym now, I'm doing a 3-5 minute warmup on the elliptical at just enough of a pace to get some light sweat forming on my head. After that, I do 30 bench presses, 30 lateral bar pull-downs, 30 squats, and 3-30 second sets of planks. I don't worry about isolation exercises because they're not worthwhile to me, and they won't be to him because there's really no point in toning one specific muscle when they all need equal amounts of work. Any of those exercises, including the plank, can be modified to be easier for him. If he's not at the gym, those easily translate to bodyweight pushups, pullups, squats, and planks, which can all be modified to be easier at home too.
If your friend has a gym membership, have him get in there on an elliptical. After he adjusts to the movement the first few times, he'll feel great and it will give him a full body workout with little to no strain on the knees and ankles. Like I said, it's hard to get used to the movement, because it is so different from walking/jogging, but it's worth the adjustment. He could also try a rowing machine or a stationary bike. Stair-steppers are okay, but they can cause a burning sensation in the joints after a couple of minutes.
If he doesn't have a membership, tell him that walking should be fine. Any movement is going to built strength in those joints and it will become easier by the day. Something I learned from the guys here was to take it to the grass. It's much more forgiving than pavement/concrete/blacktop/ect so the joints don't take as much as a beating.