I too have suffered from knee problems since I was thirteen and grew six inches in one year !

There are a lot of good lower body exercises your friend can do.

• Leg Extensions - as you mentioned are a great place to start
• Hamstring Curls - If she is getting a weight bench for leg extensions she should be able to use the same bench to do hamstring curls
• Calf exercises - This is really easy and you can use your bodies own weight to get resistance. Stand on an elevated block (i.e. an aerobic stepper) and put your weight on the balls of your feet with the foot hanging off the block. Raise and lower your body. This really works out those calf muscles.
• Lunges - are great lower body exercise but can put a strain on those knees. I do try to do these but whenever I feel my knees crying out in pain I'll stop.

There are also a lot of leg lifts that can be done laying on the ground. For low-impact exercises yoga is a good place to look too. I'd also recommend abdominal exercises (yuck - hate those sit-ups, but they do wonders).

There are tons of books out there that can detail low-impact exercises for people like your friend and I. The best thing to do is to listen to the body and stop if it become painful!

Kiks