I get that it's her mission to protect the 2nd prince, but they don't know that. If they suspect her of being an accomplice, to task her with finding the cuplrit is naught but folly for reasons twain:
1. Burden of proof lies in the accuser (then again, snob nobles oft flaunt the skirting of law)
2. If she truly is an accomplice, she'll have leave to cover up evidence, foist blame on an innocent, etc
They're better served investigating themselves but also securing Monica's cooperation just in case. Then again these are youths, at the height of summer foolishness.