Hi hh
No the DWP did not have to make him fill in the form ( indeed they don't always send them out) and it is not their role to refer him to the CAB. I would, however, see if you can find out whether he is on the system as a vulnerable person because there are a special team to support vulnerable claimants. You will have to push them, if you phone, and even if he isn't you can inquire why not, or what criteria are needed for him to be so. Always ask for a call back from a supervisor if you're not getting any joy from the first call handler you get.
BIS
Nothing on this board constitutes legal advice - always consult a professional about specific problems