The way I understand the law (I am a small-scale landlord myself) is: after the rental agreement was made (no mater written or verbal) it is a binding contract. However, the potential tenant is not a tenant yet. On the move-in date the landlord is required to let the tenant in the apartment and only after that the tenancy begins. So, potential tenant cannot simply move in if the landlord does not allow it, but, at the same time, the landlord is required to allow the tenant to move in. Consider a following hypothetical situation:
1) Your lease starts on April 1
2) The existing tenants must move out on March 31
3) However, the existing tenants did not move out and still occupy the premises as on April1.
You cannot simply break in - it is a felony. Moreover, the landlord cannot simply call the police and evict the past tenants: the eviction process is quite long and it should go through the tribunal. So, in this situation two parties are in violation of their contracts: past tenants (who did not leave in time) violate their rental agreement and the landlord (who cannot let you move in) violate the rental agreement with you. In ideal world, past tenants will be evicted withing a few weeks, the landlord will pay for your hotel (or offer you a temporary housing) for this time period and then you move in. After that, past tenants will compensate the landlord for the expense he uncured in order to accommodate you. Unfortunately for landlords, past tenants usually insolvent, so, all the money comes from landlord's own pocket.
So, your GF will definitely get her money back and then some. If the landlord states that he will not let her move in, the best course of action is
1) Insist that landlord honour the rental agreement
2) If he clearly indicates that he will not honour it: find an alternative rental of similar quality and then go to tribunal asking for the return of the original payment, any rent differences between the agreed rent an the rent of the new found rental (if any) for the entire duration of the lease, any cost of temporary housing (hotels) if such housing was require, any additional moving/storage costs.
If, indeed, your GF damaged the property, he will be able to go to the courts and request the compensation for the damages. This is completely separate issue from the tenancy agreement.