First, all good advice. But I would add in that it may be a good idea to see a physical therapist to check on any potential knee injury you may have.
Second, it may be that you may need to STRENGTHEN the knee. Sometimes, one knee is stronger than another, and doing weight training activities to strengthen the muscles around the knee would help to strengthen the knee itself, which would make it less injury prone. Sooooooo... spend some time working with the leg press machine, squats, isolated leg lunges, hamstring curls and quadricep extension, and using the adductor/abductor machines to exercise the legs. For the hamstring curl, quad extension, squat, leg lunges, and leg press, try to exercise one leg at a time. That way, you fully exercise each leg separately, which negates the possibility of the stronger leg assisting the weaker leg while doing the exercise. Do make sure to use proper form when exercising, and hire a personal trainer to put together an initial weight lifting program so that you can be sure you are using proper form and are exercising correctly. Then over time (months and coming years of weight lifting), you'll be working to increase weights. Strength is built because of the ability of the body to adapt and heal from the exercise, which would lead to eventual growth. With the growth, you would need to increase the weights to continue to challenge the muscle so that it will continue to grow. Start with low weights, since you are having knee problems currently, and you don't want to overwhelm the knee by doing too much too soon. As your knee strengthens, you will be able to continue to add more weight to the exercises.
Now, at the same time, you'll want to definitely check your bike fit and your form. If your seat is too low, you will definitely cause more stress on the knee. If you can get a bike fit, ask for something between 5 and 15 degrees for a bend in the knee when your foot is at the bottom of the pedal stroke. For some, this may be too high, and you'll want to check by letting someone watch you as you pedal on the trainer. If your hips are rocking while you're riding, that means your seat is too high, and you're straining to reach the bottom of the pedal stroke, which causes the hips to rock. In that case, just lower the seat slowly, milimeters at a time, then recheck and remeasure, and watch for hip rocking. It will take some time, but you'll find the optimal saddle placement over time. Then check things like pedal stroke. If your feet are pointing down while you're pedalling, you can switch that- you want to make sure as your leg comes down in the pedal stroke (like from 1 o'clock to the 6 o'clock) that you are not pointing your foot down. I see a lot of people doing that, and it not only puts a lot of stress on the knee, it also weakens your pedal stroke overall. The power in your downstroke comes from the ball of the foot, not the tips of your toes. So do make sure your foot is a bit flatter as your foot moves over the top of the pedal stroke and heads for the 6 o'clock position. I will try to find a chart and post it here for you (I will have to scan the page in the book by Ed Burke called "Serious Cycling" where he diagrams the most optimal positioning for superior performance when cycling).
For your friend who runs, remind them that running is a high impact activity, while cycling is a low impact activity. The higher the impact, the more stress on the joints involved in the exercise. For running, it's the knees, so when the foot makes contact with the ground, the vibrations caused from the foot hitting the ground will travel up the legs and into the kneecap. This repetitive stress will cause pain in the knee over time if not exercised properly. But this vibration can be dampened by wearing better shoes too, so you may want to check with your friend and see if they have the proper shoe for running. Try Niketown, Fleet Feet, or Vertel's. They all have a good staff and can recommend a better shoe for running if you're having problems with dampening the vibrations from running, which causes the knee pain. Cycling is definitely less stress on the knee- we have the protection of the bike itself, which has (for the most part) material to absorb a lot of the vibration of the road while riding, which keeps a lot of the stress off the knees. If you are feeling some pain still, consider your gears. If your gears are too low, then you may have problems with spinning. This is primarily due to the fact that with less gears, you have less control of the wheel itself. When the speed of the wheel is faster, and the speed of the wheel is faster than the speed of the legs, that means you have very little control of the wheel. This creates a bouncing effect, and this bouncing comes from small vibrations at the pedal, where the foot will disconnect with the pedal, creating vibrations that will travel through the ball of the foot, up the knee and into the kneecap. You will notice that your tailbone is bouncing in the saddle as you attempt to increase your speed. As long as you have lost control of the speed of the pedal, this vibration will continue. The solution is to use slightly larger gears. Increase your gears until you stop bouncing in the saddle.
Conversely, using too high a gear would also create knee pain also. Mashing on the gears creates a large stress for the knees. I always tell people to watch cadence. For hills, your speed should be 60 - 80 RPM's, and for flats, you should maintain 80- 100 RPM's. 90 would be a good cadence for flats for the average cyclist. If you are mashing up a hill at less than 60 RPMs, you will have to decrease your gears- whatever it takes to take stress off the knees. You don't want that grinding of bone against bone as you mash your way up the hill, compressing the kneecap and the joints- that's causing your knee pain.
I will look up that diagram and get it online sometime soon so you can check your foot positioning while you're pedalling.
Koffee