Change to Joint Tenancy, add Permitted Occupier

Tenancy Types and Management

Hi, I have a property that was originally let to Joint named tenants, was a 6 month AST initially, now rolled over to a Periodic, started initially in May 2022.

The couple have split and she has moved out.  Approx 2 months after this happened I was notified.  Have had an email from exiting tenant confirming she has left, left her keys and would like to be taken off the tenancy.

Remaining tenant wishes to stay as sole tenant.  However now wants his current girlfriend to move in. He has been a good tenant and rent always paid on time. He paid the Deposit, it is protected in his name.

I have checked the new gf has right to reside in UK using the gov.uk service (Ukrainian national).

Questions -

1) which document format do I use to remove exiting tenant and to add new girlfriend as a Permitted Occupier?  Will a Variation agreement or Tenancy Amendment letter suffice, for example.  Is there a suitable NRLA template?

2) Do I need a more formal valid notice to quit or deed of surrender from the exiting tenant?

Thankyou