Can one stop being a garantor?

[deljoo]

My friend had acted as garantor of his friend for the lease of a shop. He even paid the last three months of the rent for the shop although his friend had gone on holiday. He has now found out that his friend is not coming back to the UK and has got married and has no intention of paying him back. He has no forwarding address for him and the landlord obviously takes him responsible. My friend is going mad. Can he just write to the landlord and say that he is no longer that so called "friend's" garantor? or are they consequences for my friend?