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If yours is the only name on the Lease, let the landlord know that you are not interested in renewing for the next year, and that once the Lease is up you will be leaving. That way the landlord can start looking for a new tenant. Let him know that your friend and his girlfriend have been staying at your place, and that they have expressed an interest in having the place to themselves. Then they can apply for the apartment and submit to the credit check, etc. If they are not on the Lease, then they have no greater claim to staying in the apartment then someone else who is interested in renting it.
If your friend's name is also on the Lease, then let him know that you will be telling the Landlord that you will be leaving at the end of the current Lease term and will have the Landlord contact your friend in order to have the Lease put solely in his name (and that of the girlfriend, if that is what your friend wants).
Whatever situation you find yourself in, do not leave the apartment without making certain that your name is no longer on the Lease. Otherwise, as others have written, you would be liable for unpaid rent and other costs should your friend and his girlfriend not pay the rent.
Also, make certain that all your communications with the Landlord are in writing, and that any conversations with the Landlord are also committed to paper. Also copy your friend on any letters to the Landlord.
Finally, if the utilities are also in your name, make certain that they are turned off when you leave, or transferred into the name of your friend and/or his girlfriend.