She needs to see a solicitor who specialises in this area.
Notice to evict will have to be served, usually a section 21 which is a 'no fault' termination of the tenancy or section 8 if, for example there's un paid rent. This is the case for England.
The notice served will give the tenant notice to leave, which is 4 months with a section 21 but, if they refuse to leave the matter would have to go to court and it's notoriously difficult to get an eviction.
We are in a very similar situation. A beautiful one bedroom flat which has become a sh*t hole. It's filthy, filled with clutter and the smell is off the scale.
We've served a section 22 and luckily our tenants appear to be in the process of buying their own home so will be leaving.
Work is needed at the behest of the council for internal wall insulation but as it's impossible to do this with the current state of the premises, and the tenants making life difficult for our builder, the council has agreed for the work to be put on hold until they leave.
If, for whatever reason they haven't left by 4th January 2022, we wont be taking legal action because it cost in the region of 5K to 10K.
They are paying rent so for us, they can live in their filth and have the inconvenience of building work while living there.
It's little wonder that the number of private landlords is declining when it's nigh on impossible to evict a tenant, even if you want to live there yourself, and the cost of doing so tuns into thousands.