S21 issued, one tenant vacated not sure on the second tenant?

Hello, I’m trying to help someone who has vacated a property that is on a joint AST. No notice has yet been served by the tenant who has vacated but the agents are aware of the situation. The tenant who remains wants to take on a single AST but has failed the checks and cannot provide a guarantor so has been refused. The landlord has now issued a S21.

My question is should the tenant who has vacated give their notice to coincide with the date that the S21 expires? I am concerned that the 2nd tenant will not vacate and that the landlord will need to go to court to gain possession and want to ensure that the 1st tenant is not implicated in any court proceedings.

The 1st tenant does not want to give notice any earlier than the S21 date due to previous threatening behaviour of the remaining tenant which has resulted in a case being lodged with the police though no further action was taken.

Thanks for any guidance,

Alison