Yoga Teacher Question About Pre-existing Knee Problems

At the moment, one of my Yoga students has a pre-existing injury to his knee joint caused by football in February this year, and there still is fluid on it to this day. I am not exactly sure what to do with the knee. Yet based on my understanding, hamstring stretches and quads stretches will be useful for him, as well as advising him to elevate his knee higher than the heart, as much as possible.