She can break the lease but the landlord will be able to come after her for remaining rents or contractual damages. Many residential leases have terms for breaking the lease early, which includes paying a lease break fee. Unless the landlord violated some duty to your daughter, the fact that a criminal act occurred on the property is not of itself grounds for getting out of the lease obligations.
Adding to what Mr. Fife stated, even without the break fee, you could still break the lease. The landlord then has an obligation to mitigate his damages, that is, get the apartment released. If the landlord leases the apartment for more money than your daughter is paying, she will not owe any money for future rent. However, there is still the likely cost for rent until the apartment is released.
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To echo what the other responders said, it is possible for your daughter to break her lease. The only question is what she could be liable for after she breaks the lease. The first step is to check the lease to see what provisions the lease makes for lease breaking.
Sometimes it's a relatively small fee charged in exchange for breaking the lease without any additional consequences.
It's always possible to negotiate with the landlord as well and help the landlord find a new tenant in exchange for getting out without problems.
If she truly fears for her life, it sounds like there's no question that she should do whatever she feels necessary to protect herself and her son.
Here's a website compiled the University of Arizona's law college library staff. It contains quite a bit of information and some resources that might make the process easier and clearer.
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