Unapproved dual occupation, who has to move out?

My sister in law is living in a house that was illegally subdivided and somehow the council got wind of it and has issued the landlord with 90 days to rectify.

The other side has the landlord's son living in it.

The landlord has now told my SIL that she has to get out.

Am I right in assuming that that's not necessarily the case, if her lease extents beyond that and if the son moved out, it might be compliant (as long as there no kitchen in the other, smaller part).

 

Even if she has to move out, she probably has a cause of action for her losses and the inability of the landlord to provide the rental accommodation for the rest of the fixed term.

 

As well as claiming compensation for all of the rent paid for the entire term if it was an illegal tenancy.

It will be cheaper to kick out the freeloading son and get rent from your SIL at the same price as her existing tenancy.