Tenant does a runner

Tenancy Types and Management

Hi, this must have happened to people before...

Over a year ago I had a couple move into a flat I rent. They are both named on the AST.

A short while ago the guy did a runner on his girlfriend. She is still at the flat.

Where do I stand with the AST/deposit etc.?

(a) His name is on the AST so I assume he is still 50% liable?
(b) The deposit (lodged with the DPS) is in his name only. There is damage to the property plus rent arrears so I will be wanting some or all of the deposit towards costs - but I have no contact details for him..
(c) Who do I serve notice to? I presume still both of them (even though he won't know anything about it)

Thanks