GSTR1 Quarterly basis

Dear all, one supplier filing GSTR1 on quarterly basis, so we can't avail ITC at the same month as per rule 36(4). shall I ask him to file monthly or not. my view is those who are having turnover below 1.5 crore GSTR1 Quarterly basis is only optional not mandatory. so we are paying tax on monthly basis then we can ask him to file on monthly basis, otherwise we will pay the tax amount only when it was reflect in our gstr2A, is it right or not