Do you know whether she is renting privately or from a Council/Housing Association. If it is the latter then she would need to approach her housing officer and advise them of the situation. As they are indoor cats and if she can guarantee (well, as much as possible) that they won't do damage to the property then there should be no problem with her keeping them. It would be a slightly different situation if they were allowed outdoor access AND it was a shared entrance (eg in a flat) AND they were dogs.
If it is a private rented situation then there is some debate over whether a "no pets" rule is enforceable in law. Tenants are entitled to quiet enjoyment of a property and a "no pets" rule breaches that if the leasehold states no pets. Or something like that - would definitely need more research to be sure though. I daresay this could be applied to council/housing association properties too. I haven't got all the links on this computer but can have a look tomorrow if that would be of any help.
In any case, a landlord cannot enter a property without giving the tenant written notice, again, part of the quiet enjoyment of the property. The point being, if she decided to keep the cats, the landlord, in theory, wouldn't know unless they were very noisy or the neighbours complained and she would have plenty of time to temporarily "de-cat" the house if the landlord needed access.
Karen
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