Put a paper plate under her drag foot and drag it up. Never did this but I have heard our PC tell girls to wear a heavy boot to feel the drag. Also, pitch with a hole in front of the rubber and try to drag. I only used the paper plate and that helped my DD.
Our pitching coach had DD do the paper plate drill. Not really to correct a flaw, but to demonstrate early in our tenure with her, how the foot drags.
Probably a decent exercise for all pitchers to do, just as a check/reminder.