Hi, thank you for your question. If you are both on the same tenancy agreement you will be considered joint tenants and either of you will be free to give notice to end the tenancy agreement without requiring the consent of the other tenant. Your friend will need to contact the landlord and negotiate a new tenancy agreement.
It is usual for joint tenants to be jointly and severally liable for the rent - therefore the landlord can pursue either both of you or either one of you for the settlement of the rent until the agreement ends. You would therefore be liable for the rent if your friend does not pay.
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If you give notice the tenancy will end and he will need to speak to the landlord about obtaining a new tenancy. IF he stays after this time the landlord will need to apply to court to evict him as he will no longer have a right to remain there.
You are both liable for the rent jointly and individually, but if you give the necessary notice you will not be liable after it expires.
If your fixed-term agreement has ended and you have not signed a new one then you giving notice ends the tenancy agreement for both of you. The estate agents are incorrect. I refer you to the Shelter website here which reaffirms this position under "ending a joint tenancy": http://england.shelter.org.uk/get_advice/private_renting/private_renting_agreements/whose_name_the_agreement_is_in