RCM ON INWARD SUPPLIES paid in March 25

My client purchased goods from unregistered  supplier and paid RCM on 1st March 25 now he is filing GSTR 3b for February 25. Shall he show purchases in March 25 and claim on RCM in March. But Cash ledger showing RCM balance  for March. My query is cam he file nil return GSTR 3b