B.F.
Being good is not all what it's about and as time goes she will get better. It's first about disaline, he should understand every one gets different belts at different times, every one masters their level based on their own ability she's not going to learn anything by being stopped because of her ability. In fact if he does this he will only add to making her self esteeme go down hill and she'll never forget it, it could effect future things that she tries.
He didn't make her stop coloring when she went outside the lines? well, then so she's out side the lines on this she won't be forever. Stick to your guns and tell him it's not just about now it's about later in life how doing this will effect her.
Best of luck