The legal position is that your friend is solely responsible for the entire rent and satisfactory fulfillment of the contract until the end of the very last day of the lease, whether he uses the flat or not. There is no escape from this, he will not be released, and should he flee he will undoubtedly be pursued with vigour to a very costly ending. However, he has the valuable benefit of the "head" lease, and it should be possible for him to find someone acceptable to the landlord to whom he can sub-let for not less than the amount he is paying and on the same or improved terms. But, and I cannot over emphasise this, if the sub-lessee defaults then the situation reverts to the head-lessee, your friend, being liable for the full amount of the rent until the end of the of the lease, plus repairs etc.