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That early in the game, you are doing no damage as long as she can play the parts without noticable "strain". The mouthpiece is too big if she has to squeeze the notes out.
Whatever you do, try not to infect her with the whatmouthpieceisbest syndrome. There is no cure, it is very expensive, and has ended many a trumpet playing career.
I do not personally subscribe to the Lindemann school of thought. Amateurs often need the advantages of a larger mouthpiece to compensate for inconsistencies in their daily playing. The larger "sweet spot" on larger mouthpieces aid just that. Professionals and advanced amateurs practice enough to make just about anything work.
At the end of the day, her playing will determine if the bigger mouthpiece is a blessing or curse. My students usually stick with a mouthpiece for at least 2 years and I only bring up the subject if I feel that there is some benefit. Sometimes they ask what they should do and I ask in return to what degree they are willing to commit. Without commitment, any expense is wasted money.

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Whenever I feel blue, I start breathing again.