Late payment of rent & dispute between joint tenants

Rent Arrears

A couple who rent my flat have an AST in joint names. one month into the tenancy, one of the couple decided to leave. The other tenant wished to remain in the property but she said her ex partner would continue to pay his half of the rent for the duration of the tenancy. She pays her half on time but I have been left to chase her ex partner for the rest of the rent. I advised her that if she is experiencing difficulties she should share the flat with someone to help her pay the rent (it is a 2 bed flat)but in the meantime I expect the full rent payment on time and one month in advance as per our agreement and that it is not my responsibilty to chase any outstanding payments from her ex partner. I have since received a message from her ex partner advising that he will pay his half at the end of this month(this make his half of the payment one month late). What action should I take and what is my legal position in relation to this?