you have to be careful with that one. Depending on what kind of surgery she had, even isometric exercises may be contraindicated. I would advise her to check with a physician or physical therapist.
If she is cleared for exercise, I would start with isometric contractions, then hip exercises and straight leg raises and heel slides.
I’m with Karin on this one. You can also have her sit on a table/chair/bench and have her do some leg extensions. She can start doing them without any resistance and then once she starts getting stronger you can add some resistance with a band and then progress by adding an ankle weight and then maybe move to the leg extension machine. Of course a PT could give you some more ideas/recommendations. I hope this helps.
In addition to the above advice, you might want to know what part of the knee was injured, the nature of the surgery, and then assess to see if there are any other issues going on with her above or below the knee joint. For example, if her injury was to the medial meniscus she might need work on the vastus medialis but also on glutes to help strengthen external rotation.
The above info is great.
From my personal experince working with post-rehab clients, such as ACL,MCL, torn achillies etc.
It all really depnds on the clients fitness level before the injury.
And remember to always get a doctors clearance from the client before starting any physical activity.
All the best and good luck,