UK eviction notice's?

hi, my fiancee's mother has 2 houses one she is living in and the other is a buy to let ( house that they can only rent out) her husband ( going through separation) is living there at the moment and hasnt paid the rent for over a month and the house is also up for sale along with the one she is living in which has also just sold. She now wants and eviction notice sent out but is worried it is going to take 3 months when she wants him out the house so they can actually sell it but he wont move out. how can she go about doing this through and eviction notice as he isnt suppose to even be living there in a buy to let property