Work with various feet widths. While her toes should be pointed slightly out, also remember to "spread the feet" as she comes down in the squat. This will work the hips a bit more. Have your girlfriend try squating w/out weights, even bar. Just work on the form at home even.
Try box squats. And if you don't have a box, like us; we stack step benches or use a bench that allows us to go just below parallel. This will show your girlfriend how the hips are so important in squatting. Remember: hips, hips and hips! And glutes and hams. How's her glute and ham strength?
Oh, and I'll be wearing good old fashioned, Chuck Taylors (Converse) for squat day. They keep my feet flat. The other option is to squat bare foot. Don't think my gym would let me do that - bastards. Anyways, make sure her shoes ain't elevating her heels.