1)If he/she under going of his/her Articleship or already completed then it can trace with the help of TRACE A MEMBER available at www.icai.org by using the member details.
2)Otherwise it can trace if any documents related to registration available. Or any place if it noted.
3)If there is no way to get from our side i.e. no documents available with us which are indicating the registration no in such case i think write to ICAI where it would be register by requesting send a duplicate registration copy. However i am suggesting you post your query at e-sahaayataa available at www.icai.org post your query as others and follow the suggestion of ICAI. OM SAI SRI SAI JAI JAI SAI