Chris
You’re rather short of replies so I’ll offer my thoughts.
The l/lord is clearly in breach of the tenancy agreement & his duty to the 2 tenants to allow them quiet enjoyment. Your claimant should be advised to seek legal help – CAB, tenancy relations officer if you have one at the council.
Nevertheless, currently the claimant still has an agreement to pay half the rent. I presume the eligible rent ifor HB is restricted because previously over accommodated. I would therefore get a new ROD (2 tenants & 2 other occupiers) & assess on the new decision. I don’t think that the decision to assess on one third liability is correct. I would make sure payments go to clmt & if he wishes to withhold rent, that will be his decision…