Ill add what I can to the already great advice you’ve received.
If this is an acute injury and inflamed I wouldn’t focus to much on strengthening or stretching but allow for any inflammation to dissipate. Although there is a current debate over ice or not, I always say a little ice and rest never hurt no joint. 🙂
First, I do not work for the following but I highly recommend you look into Foundation Training. www.foundationtraining.com It’s a great physical concept and program for shifting the body to more of a posterior dominant line. It will help you bring the client to a place of more glute control and a lengthened hamstring. Remember, flexibility is meaningless if you can’t recruit properly. Think effective length rather then flexibility. Most exercises can and should be done standing.
Also, I like to think of the knee as a dumb joint. It only does what the muscles around it tell it to do.
Lastly, look for a Physical Therapy platform. Essentially its a large raised gym mat that you should be able to get your client on, allowing you to do “floor work.”
I hope some of this helps. Please feel free to contact me with any more questions that might clarify my answer,