Originally Posted by BPinard

My best advice (I know I'm not a teacher) would be to tell her something along the lines of:

"Even though buzzing your mouthpiece helps a litte bit, it can't make you a great trumpet player. In order to get better, you have to practice with the mouthpiece in the horn. So far, you're practicing playing your mouthpiece. In band, you don't play the mouthpiece, you play the mouthpiece AND the trumpet."

I know that some people might disagree with what I'm saying there, but I'm sure you know what i'm trying to say. Just let her know that she'll get better from practicing with her trumpet attached.