I had a 3 year old Hanoverian lock his stifle at the canter and fall on me when I was backing him, he has since had his stifles injected and is showing fourth level dressage without a problem. He had always lived outside before he was imported and had daily turnout once he got here. Locking stifles can happen to any growing horse.
The problem is that the more they lock the more they lock. If it ever locks while she's standing up, BACK her up, don't let her walk forward and "Pop" it.
If you do any sort of work with her, make it straight lines, don't lunger her on a circle. I was told by a vet that driving a horse in a straight line either through the snow or with a small ammount of weight would help. I would ask the vet specifically what he suggests you do to make the muscle stronger.