One tenant wishes to end agreement

Rent Arrears

27/07/2009 - Mr and Mrs sign 6 month AST for house.

25/08/2009 - Mr and Mrs seperate (Mrs has no income).

10/12/2009 - Mrs has successful claim for LHA.

25/04/2010 - Mr sends letter stating he no longer wishes to be a joint tenant of the property.

27/07/2010 - Rent arrears stand at £600 (1.5 months)

1.Can Mr legally end his involvement with the AST ?

2. If I decide to evict Mrs what timescale do I have to use ? None is stated in the agreement and I issued a S21 notice on date of start of AST.

Thanks

N.B. The rent payments have been intermitant throughout the whole life of the agreement.