one tenant moving out (roommate situation)

[Lyana Yonk]

Two friends rented my apartment a year ago, one announced to me that she is moving out at the end of the lease. The other said she wants to stay and she is looking for tenants. What if she doesn't find tenants? How to make sure she is actually looking for one. The tenants moving out announced that a while ago and if I was the one searching I would have started right away. The tenant who want to stay told me she might have some friends interested but I doesn't seem like it worked out. Something else she decided to switch room and take the room of the tenant moving out and is trying to rent her room (less desirable). I feel like it will be hard to find a random tenant and I always rented the place to friends together or couple and one never moved before the other one. I want to make sure I have my full rent in one month when the other tenant move out, how can i protect myself? Worth saying that if I was to rent the place fully empty with no tenant, I would have no problem finding tenants immediately. So I am not sure the tenants staying should be responsible for the rent if she doesn't find anyone.

Btw they have a lease together.