return return filing method - below 20 lacs turnover

sir I am a GST consultant I have one doubt in GST return filing. one of my client has below 20 lakh turnover per annum. he is showing purchases and climbing Itc in GST 3b return but but he has not collected tax from his buyers. so he has not utilised Itc as he has not collected tax from his buyers. he also has shown his turnover as exempted GST 1 return and gstr 3b return. during the last month March of that financial year he has revised all utilised ITC. this method of filing is correct or not. one of my colleagues said it is a wrong method. he told me that every registered dealer must collect tax and adject tax with ITC so kindly clarify my doubt thank you very much