Can a landlord w/hold security deposit on a tenant that is leaving because the new tenant will not be renting?

My mother recently moved. She gave a 30 days notice to the landlord of the place that she moved from. Her lease does not expire until December but the Landlord, was OK with it and signed a lease with another tenant, to move in when my mother vacated. The new tenant got drunk and showed his butt around the neighborhood so the landlord cancelled the lease with the new tenant and is refusing to refund my mothers deposit because of this. Doesnt this new lease void my mothers lease? Does anyone know where I can find these laws??