anything seated is better than standing or walking too much as it unloads the knee joints. If there is a pool available that would be great.
In addition to the already mentioned exercises you could try to do a “sitting-down/standing-up” (or box squat) exercise with a stable chair or bench that should be high enough. To facilitate this move he or she could use a TRX, ropes or strong bands to hold on through the move. This is a good way to teach how to use hamstrings and hips (pushing the butt back) when sitting down and standing up which all of my clients with knee problems can do.
I don’t have a video, but hope you can make sense of this.