Depends. Not signing it doesn't necessarily mean she hasn't accepted the terms of the contract.
If she received the contract a few days ago, and hasn't signed it, then fine. If she received it months ago, didn't sign it but didn't protest about the terms and conditions, instead just carried on working as normal, then her employer could say she accepted the proposed terms.
If she received it, didn't sign it and notified her employer that she wasn't going to sign it, and wasn't happy with the terms and conditions then that's different.
once she's actually started working then she's accepted the terms and conditions of her employment by default as it were, unless she's actually recorded a protest to some part of it.
if a suitable replacement for her can be found quickly it may be that her employer will negotiate with her so she can go early. if not and she goes anyway she could be deemed in breach of contract, which may cause problems with future references.