Just want to chime in on this since I have been told I am a prime candidate for the surgery Ball and Landy had. My quadriceps tendon was completely severed off my patella when I shattered my distal femur in april. I ended up losing a lot of cartilage while my knee was open to the world. Landy and Ball's injuries were much less intense than mine, but I understand what kind of recovery they are going through a little better than most. I have been told that even in the best case scenario I will need a cartilage or knee replacement in the future.
My best guess is Landy severed his Quadriceps Tendon directly above the patella during the skate incident with Makar. Something went badly after that injury/surgery and between a lifetime of cartilage degradation from being a pro athlete/the new injury he developed something similar to what your girlfriend dealt with. The traditional way of fixing that is a knee replacement, but since Landy is a professional athlete and his injury was mostly limiting him from performing at a high level, he opted for the experimental surgery instead. From my conversations with my surgeon, (who has performed a few of these are these cartilage replacements) if it works correctly the patient can get back to 90-95% of the level they were at before, a knee replacement is more like 75-85% at best, you're always going to be limited somewhat like your girlfriend. The only thing we can do now is wait and see how Landy recovers. He really could get back to professional hockey if everything works out, how effective he will be is difficult to say.