How do you train a client with a history of quadricep tendonitis & patellar tendonitis? What exercises are safe, which avoidwhy?

I wouldn’t not do squats or lunges. Maintaining strength in the glutes are critical to alleviating destructive forces on the knees. You can reduce the range of Morton, ensure the squat is starting with a partial sitting motion then extend the hips from there. No need to go deep our full squats, but we squat to sit, stand, perform daily activities, so we have to continue to strengthen those muscles performing those motions. Lateral motions could strengthen those stabilizers that are the hip abductors and adductors.