Even during the fixed term of the tenancy the landlord can still apply to get the tenant evicted under section 8 of the Housing Act (rather than s.21, which can only be used when the fixed term is up). (Whether she wants to is a different matter.)
Text below comes from here.
If, having tried alternative means of resolving anti-social behaviour being perpetrated by a tenant, you feel that you have no option but to seek possession, there are grounds for you to do so under section 8 of the Housing Act 1988.
Ground 7A, is for cases where serious anti-social behaviour has already been established in other court proceedings anti-social behaviour has a minimum notice period of 4 weeks (for periodic tenancies) or 1 month (for a fixed term tenancy).
Ground 7A is a mandatory ground, which means that the judge must grant possession if you can prove that the ground has been met.
Ground 14 is for nuisance or annoyance, or the illegal or immoral use of the property, and proceedings can be commenced immediately after the service of the notice.
Ground 14 is a discretionary ground, which means that you need to bring forward evidence to prove that the anti-social behaviour has taken place and, if the court is satisfied by the evidence, it will take all circumstances into account and will decide if it is reasonable to grant possession.