The answers are great. I agree with Karin to consult with the client’s physical therapist to determine the extent and history of the patellofemoral is essential prior to beginning the exercise program.
I would suggest high impact excises, like running because the p-f history. Rather, eliptical, recumbant cycle, cycling outdoors. will probably be the best.
I encourage intervals to ventilation threshold. Speed up to the point where your client begins to lose his/her breath, then slow down until he/she has caught there breath. Your client can complete 6, 5 minute intervals in 30 minutes and will most likely will be less fatigued than in doing 30 minutes of sustained pace. Pushing to threshold is important because that’s caloric expenditure is accelerated. However, most untrained to moderately trained people can’t sustain threshold intensity for more than a couple of minutes due to the increase in lactate.
Good luck, take care.