Income tax applicability

My mother conducts tuiton at home and earns roughly around Rs 12-15000 p.m. Also she gets interest income from FD. She invest for the full amt in PPF i.e. 150000 from the interest on FD and deposits her income in cash monthly into the bank.Now she had submitted form 15G to claim exemption on interest of one FD worth 10 lakhs and on another FD for 10 lakhs which we decided to keep during the next month we didnt submit 15G for this other FD as we wanted to file returns and the interest amount would anyway have gone above the tax exemption limit. One question that I would like an answer to is when we conduct tuitions at home, and it is not a full time buisness. How do we prove that the income generated is indeed from tuition.Also will the part submission of 15G cause any problems?? Will I have to break up the portion that was taxed and give declaration for the one untaxed??