Assured Shorthold Tenancy Help

[Guest]

I write on behalf of a friend in the UK who has an assured shorthold tenancy. His landlord moved in as they developed a relationship but has moved out again with another partner. He says he wants his house back within a couple of days. My question is does the Tenancy agreement still stand as it was never ended ( he has made one payment since) or as they are saying that they were cohabiting and therefore tenancy is voided. Any help appreciated thanks