Interview under caution

Hello. A close friend ha received a letter to attend an interview under caution at the dwp because he failed to notify them of his recent pension payments. He mistakenly didn't realise this was necessary and still doesn't understand what he has done wrong because the rules are so complex and he is so unwell. He doesn't understand why he may be on income related ESA and has been on it for several years. He has over 30 years full NI contributions so I can't explain it to him either. He is in the support group but recently became entitled to his pension and now the dwp are threatening him but he doesn't understand what he may have done wrong. Any help or advice gratefully received. He is out of his mind with worry which is not helping his condition at all.