About sc certificate

My friend is from westbengal and his surname is saha. Now most of the saha surnames belong to Schedule Caste and he has no proof of his own caste, so he went to his local municipality and they said u can choose whatever caste u think u were and to get a caste certificate u have to get a copy of caste certificate of a same surname's person with the same caste u mentioned...he got a Saha person with Schedule caste and he gave him his documents and the local councillor also give him a certificate saying that he also belong to the same schedule caste....and based on those informations the SDO also made his caste certificate mentioning him as a schedule caste....now is it wrong or right ?? As he has no prove of his own caste can he obtain a Schedule caste because most of his surname persons belong to SC ?? He has no mentioning of his caste in any documents so only based on public hearing and the certificates from mla councillor and chairman , the SDO made him the caste certificate