Current tenants mum moved in at start of lockdown

Tenancy Types and Management

One of my joint tenants -of the last four years- moved his mother temporarily, into the spare bedroom at the start of lockdown Feb 2020. He wants the move to become permanent and has requested a new lease with her name on it.
I have no objections to this but, I don't know what type of lease this would require, or if this would require any extra management from myself. Your advice would be appreciated .
The tenants themselves or from Rumania, I have all the correct documentation from the start of their tenancy. What, if any documentation would I require from the tenants mother ?.