Dual national treated different to single national in the benefit system

Question

I know somebody who put in for a claim for his child's Disability Living Allowance (DLA). His child is a dual national, they asked for a copy of the child's passport. The staff confirmed it is because they are British and another nationality rather than British only. My argument is they have no right to do additional checks based on just the fact it is a dual national child and for no other reason, a dual national is regarded British just as much as any other non-dual national. How should he continue? Does anybody know why they ask for additional documentation? two staff have confirmed if it was British only, they wouldn't be asking for this.