Hi
When you say sublet, do you mean they are renting a flat from someone who is in turn renting from the flat's owner?
In which case by landlord do you mean the flat owner or the person they are effectively renting from.
This scenario could be a nightmare if the original tenancy agreement does not allow subletting.
The reason I ask is subletting can also mean the owner of the flat, who is not the freeholder, renting his flat, in which case he may need approval from the freeholder.
When you say the rent is paid directly to the landlord, by whom? Can she not ask whoever is paying the rent to intervene?