Tenant wants to leave early and told me to use deposit for this months rent

Tenant is 6 months into a 12 month AST and has got in touch with me to say she has just been sacked and so cannot afford the rent any more.
She wants to terminate the contract but intents to use her deposit as what will be her final payment(due 7th Jan).

I have agreed to let her terminate early but how do I go about keeping the deposit as its held with the DPS?

Any advice please