Cash Deposit in Savings Account

My mother has deposited 9lakhs cash in her savings account during 18-19(she is a housewife and doesn't files return). Now she has received notice from income tax dept for not filing ITR. She has saved some money from rent rcvd from her house and some are personal borrowings from relative but she has no proof of either as they are rcvd in cash . How can this issue be tackled?