I remember learning the snow plough and it was not easy. There are a few tricks that help. Firstly learn a one foot snow plough ie only turn one skate. Do that on each foot in turn. Secondly rotate the foot about the ball, applying less pressure at the rear. And note that the skate blade needs to be at an angle to the vertical, with the bottom of the blade furthest from the skater. Lastly, do not try to stop, but concentrate on turning the foot, and then gliding along in that odd position. Forget about slowing down, just get used to moving one or both feet into a strange position. Once the skater gets the hang of that, they can start to apply downwards pressure on the braking skate.
I've taught several people to brake, and the last point, not trying to stop, helped them a lot.
It is possible the skates are 'too sharp', by which I mean the hollow is too deep. However, if she is young, she will be light, and that makes the skates dig in less. They could be sharpened incorrectly, but I'm not convinced that would prevent her doing a snow plough.