Couple who have separated, one party wants the full deposit

Tenancy Deposits

Hello all,

I have rented this property to the couple for 3 years now. Recently they have decided to go there separate ways, leaving the Miss in the house who is now paying the rent.
As far as I was aware, the deposit and rent was paid by both of them equally.
The gent who left the property is now demanding a full return of the deposit as he is the one who laid the funds out.
Is this my issue to sort between both parties?
I have spoken with the tenant and she states that they didn't split it equally there wasn't a split divide of who paid what but he did not pay the full deposit.
I am a little unsure of how to respond to paying him the full deposit?
Any advice would be much appreciated!

thank you