What do we need to settle before my flatmate gives notice and I try to stay?

My flatmate showed me the email she was about to send: "I am giving notice to leave in two months." She thought it would remove her name while I found someone else for her room. I thought we had time to talk to the landlord first.

We share an assured periodic tenancy in England, with both names on the same agreement. She is moving for a new job. I want to stay near my own work and could manage her share of the rent for one month, but not indefinitely. A friend is interested in the room; the landlord hasn't been asked about a swap or a new agreement.

The [government's joint-tenancy guidance](https://www.gov.uk/assured-periodic-tenancies-tenants/how-to-end-your-tenancy) says one joint tenant can usually end the joint tenancy, which can affect the others too. It also describes a tenant swap where all tenants and the landlord agree. Neither of us had understood the distinction when we discussed the move.

How do we work out a departure that respects her job deadline and gives me an informed chance to stay? She hasn't sent the email yet, and I don't want her to feel trapped while I pretend an interested friend is already an approved replacement.