Can a person with a knee injury use the bow flex stair stepper for fitness?

Yes. The fact is, stair steppers and stationary bicycles are the best exercises for quadriceps control and strengthening. Quad inhibition after knee surgery needs much more attention than we have been giving it.
Possibly. Symptoms dictate one's ability to participate in various forms of cardiovascular exercise. Meaning, if it doesn't hurt during or after a certain type of exercise, then it's usually ok to do it. That being said, a stair-stepper tends to be moderate to high-impact and harder on the knees than lower impact exercises such as the elliptical, biking and swimming. The type of knee injury also factors in.