Horn requires a very open vowel shape, even in the upper register. She might benefit from getting her back teeth apart and her tongue down. Think AW instead of AAH.

This is exacerbated by F Mellophone, which needs to be played with more of a trumpet approach to vowel shape (more compact).

That's the challenge of going back and forth between the two. It's not only a mouthpiece difference.

Once the kids are aware, they're typically able to make the necessary adjustments.