Shari,
There are several exercises my PT gave me when I was in therapy for Patellofemoral Pain Syndrom (aka : "my knee hurts")! The basic one was a lying extension. Lie on your back, propped up on your elbows, with one leg bent, foot flat on floor, and the other leg straight. Contract the quad of the straight leg and keep it contracted. LIft the straight leg to about 45-degrees, hold for a couple of counts, then lower, always under control.Do several sets. As 15 or 20 reps become easy, add ankle weights. This exercise doesn't put stress on the knee like machine extensions or seated extensions can.
You can also do the same exercise and target the muscle on the inside of the knee a bit more by turning the toes out about 45 degrees.
Another exercise which can strengthen the muscle on the inner part of the knee (the one that often causes problems because of weakness) is to sit in a chair, and squeeze a small (a bit bigger than wiffle ball size) ball between the knees.
Another good exercise for knee strength and stability is a one-legge squat. No weights are necessary. Lift one leg off the ground, and slowly bend into a squat with the other leg. ROM may be very shallow at first, but will get better as strength develops. This is a great exercise for stabilizing the knee as well as strengthening most of the muscles surrounding the knee.
Also keep in mind that strength has to be balanced by flexibility (and by the strength of the hamstrings, which should be about 65% as strong as the quads). Be sure MIL stretches the quads, hip flexors and hamstrings (especially the hamstrings, as hamstring tightness is very common).
Depending on what specific concerns your MIL has, different exercises may be beneficial to her.