Non filing of gst returns

I have been approached by a Proprietor with his GST number under REGULAR OPTION,  he was under the impression that the GST number though taken was not applicable to him as his bussiness turnover is less than 10 lakhs being SPECIAL CATEGORY STATE so under that Impression he didnot file his returns nor did he cancelled his registration... when I checked his GST account I saw that his returns are not filed till date..  No notices were served to him by GSTN. now he wants me to settle his GST filings .......  No incidence of purchase reflects under GSTR1 for any of the months...........

So can I go ahead showing Purchase made from Unregistered dealers and paying taxes on RCM basis.............. How do I need to review his current account transactions............................

Anyother options please help...............

Kuldip C

7679241707