It is unlikely that the original landlord could have failed to disclose the COA as there cannot be any duty to disclose something that is not known to you. That rules out billing the (original ) landlord as the payee.
The overpayment is therefore only recoverable from the claimant (Mr)
As for offsettiing or recovering from the (former) partner:
There is no such thing as a joint claim for HB so the overpayment is legally only recoverable from the claimant (Reg 69(1), Reg 82(2)(b)
Reg 83(1ZA) will not apply in this case because the claimant did not have a partner either at the time of the overpayments (they had split up at the time of the COC) or presently at the time of recovery.
Reg 85A cannot apply at all.
The provisions of Reg 85A(b) to (d) are cumulative, and Reg 85A(1)(c) does not apply because HB was paid to a different person in respect of the previous dwelling
Your only target is the claimant himself (Reg 82(2)(b)). You cannot involve any third party as a potential target for recovery
