Overpaid UC HCE

Hi all
working age client lives in a HA property who has been   iro UC with a HCE

In September 2018 his local HB Office accepted that from that time   he / his accommodation met the specified accommodation criteria and client’s l/lord (HA)  claim they contacted   all tenants affected by this change advising them to contact UC to report this change to specified accommodation and also to make a claim to HB at same time to get eligible rent paid through this

- my client is in dispute with his HA that he was ever advised to take the above action

His UC HCE was only removed when his HA had to reverify his rent following a change in this on the 1/4/2019 and pointed out on the form that client did live in specified accommodation

In April 2019 client has made a claim to HB and has asked for a backdate but as a working age person this, if agreed, could only be for 1 month
UC have not yet contacted him to advise he has been overpaid the HCE from September 2018 to April 2019 when they became aware of the change to specified accommodation - but assume this will happen

Has any one any experience of UC using their discretion not to recover overpaid benefit - in client’s case   there has arguably been no loss to the public purse as HB would have covered his rent had UC not done so ( and in fact he would have been eligible for a higher HB award for the period UC HCE overpaid due to certain service charges he has which are not covered by UC)?

If UC insist on repayment how feasible would it be to apply for a DHP for the overpaid UC period?