I don't know that you can get her off the lease. However, you can get the rommie out of the apt. Then your daughter could find new roomies.
1. File charges for assualt immediately. Then obtain a restraining order. If your daughter moves back in & can list the apt as her address, the court will require the one roommie to move out. If R2 aided or encouraged R1 in beating your daughter, she can file charges & get a restraining order against R2 also. Once they are facing forced eviction by the court, they will probably agree to terminate the contract rather than have to move.
2. Sue L for breach of the right of quiet enjoyment for not evicting R1. L will either evict R1 or have to let D out of the lease. You'll have to check your state laws to see what kind of notice & time to respond you have to give L.
3. Stop paying rent & inform L in writing that she has abandoned the apt. The worst that can happen is that L sues her & you have to pay the judgment.
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This is not legal advice and you are not my client. Double check everything with your own attorney and your state's laws.