Out of curiosity, has she ever been ASM, or been on crew before? Is she also a senior? What you could do is shes not is let her co-sm the show, and say that your insuring that she'll be a better SM next year. Let her do all of the blocking notation, give her some experience in what its really like. This allows you to do what you want, allows her to feel like shes doing what she wants, and lets you insure that there is an SM who knows what they are doing next year. If she has never even been ASM, you could tell her that she isn't experienced enough, or that she could ASM the show (that is if you have ASMs). Have you found out why she wants to stage manage? It's usually not just on a whim (although it could be). You could also tell her exactly what the job entails, that could be enough to make her not want to do it
best of luck.