I don't have a degree in biomechanics or anything like that, but I'm going to go on a limb and say she doesn't necessarily have anything to worry about. I would worry if her knees caved in while she descended rather than ascending. I also think it's natural for the knees to slightly flex inward in order to activate all the necessary muscles for a successful squat (not too much -- as long as they're still within that neutral plane that's getting so much attention lately) during the drive up. If you look at some of the heaviest squats, you'll see people cave in a little here and there, but their form and technique is solid before that.
I have a couple of athletes who do have the knees come in while they're coming out of the squat, but I think it's more so of a reason that they're trying to be too posterior dominant during that phase. Yes, you'll have to initiate that drive with a lot of help from the posterior, but in my experience (limited at best), people tend to cave their knees in because they want to shoot their hips up and get more "power" from their hamstrings and lower back, essentially turning the squat into a really heavy good morning. One cue I use to correct this is to tell them to get their "hips back under." I don't say anything else, and I see their hips not only get back under the bar and see their chest rise up back to proper form, but their knees flare back out over their feet in the proper track. The sooner you can get them to get their hips back under, the more efficient they'll be, and you'll see her squat numbers sky rocket. I'm going to say she already has the leg strength to do more, so I hope this cue helps you as well. One thing I'll advise you against is forcing your athlete to vigorously push them out past her feet. Again, there's so much data going around over that discussion that I'll end it right there.
All the best,
Javier A. Sanjuan
Olympus Barbell Club
Dear God, please help me lift heavy and be awesome. Thanks. Amen.