The position is that if the tenancy agreement has been executed (signed by the tenant and the landlord and dated) then both parties will be bound by the terms of it. It will be legally enforceable against the tenant.
The tenant can only leave before the end of the term if there is a break clause entitling earlier termination OR if the property is unfit for human habitation OR the landlord consents to the student/landlord surrendering her tenancy OR she finds a reasonable replacement tenant.
The unfit for human habitation part relates to repair really - the property is in considerable disrepair then the tenant could ask for environmental services dept of the local authority to make an inspection. Were they to declare it unfit then the tenant could terminate on this basis since it would be a repudiatory breach of the landlord's obligation to provide a property in repair.
It is not the landlord's duty to ensure that all the tenants behave civilly towards each other and if they do not he cannot suffer loss as a result.
The tenant should focus on finding a replacement tenant to produce to the landlord. The landlord is under a duty not to refuse a reasonable tenant. The tenant could try advertising on www.gumtree.co.uk for such a tenant. Private landlord's use the service all the time.
If the tenant does not move in then the landlord could sue for recovery of rent and would most likely be awarded an amount of rent for the period up to when he could have re-let it to another tenant.
Sorry it could not be better news.
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