She and you will be more comfortable now. You did right to wait as sometimes they will, at a young age, have this and it will correct....but waiting gave a chance for the animal to mature and I feel you will see good results for her.
I know of an animal that was returned to the breeder for stifle lock, was cut and then shown to National Champ in halter status.....and probably still being used for breeding
No doubt, if name given, many of the forum would know this animal too.
Keep us posted on how she does - I will be interested to hear, as I had a filly done not long ago too. Hers was less invasive surgery, they just nicked the ligaments to form scarring as opposed to cutting them. Seems to be working - she was out 24/7 here but flat ground and not moving around much so I took her out to our new place and turned her loose on probably 10 acres with lots of hills. So far so good. She also will be a pet for someone, and if she continues as planned will make a driving horse. No babies for her though.