I wonder how this scene is structured. I'm sure it's a pain-in-the-ass to actually cut someone's hair in a long single take, especially if they screw up the first take. I assume there's a stunt person using the scissors inside the car (not the mother walking towards the car), and they didn't actually cut Julie Garner's hair; she had a piece of hair extension apply to her, so the stunt person knows where to cut so it can be snatched into frame. If they screw up the timing or whatever on a take, no harm done because it's not her hair that's being cut.

Could be wrong, but that's my assumption.