This is one of those cases where a person who has acted in good faith gets swept up by an anti-abuse provision because their tenancy is of a type that is more likely to constitute an abuse of the HB scheme, even though there is no suggestion of impropriety.
This is starkly illustrated by the Reg 9(1)(d) case of Sharon Tucker and Reading Council. Ms Tucker rented a property from her ex-partner; she had custody of their child. HB was paid with no problem throughout the 1990s, but then the Regs were amended in 1999 and transitional protection only lasted until the end of the benefit period. After that, Ms Tucker was caught by Reg 9(1)(d) with no possibility of any kind of exemption or time-barred expiry of the rule. She challenged the legality of such a blunt instrument piece of legislation, especially the absence of any long-term transitional protection, but failed.
Your claimant has a similar problem: Reg 9(1)(c) does not allow any way out. It’s rough justice but it’s the law.