What makes you say this is non-commercial?
If you believe this is a ltahw case (with slim to no evidence) then this should be investigated.
If they are joint tenants, why does he not have use of the bedroom?
It does not sound like he got the best end of this joint tenancy; why does he not get less rent to pay than his fellow “joint” tenant?
Anyway, the Rent Officer will give a low valuation for this “dwelling”, if he is not a joint tenant and has use of a sofa only (need to check that these are the actual terms, not just his say so).
Are your suspicions aroused purely because of the gender of the occupants or the sleeping arrangements?
It is possible that although both names appear on the tenancy agreement he is actually a sub-tenant or not liable at all, some agents/landlords include all adults on tenancies.