Your roommate may ask to be removed from the lease, and if you and your landlord agree to this change, then the roommate will no longer be responsible for any of the rent. This new agreement should be in writing and signed by everyone to avoid later difficulties.
As a tip for next time, it is often a good idea to have a roommate agreement in place with all of the people you are living with. The agreement should specify how rent will be paid, and for how long each roommate remains responsible to pay their share of the rent. You can also specify what will happen to rent payments if a roommate leaves. A sample roommate agreement can be downloaded here.
You can view our Shared Accommodation tip sheet here.