Dear Arun Kumar Ji,
Call " Mr.X" in the witness box and Examine/Cross Examin him, ask questions about his Family, Father, Forefathers and Foremothers. If he is just Family Friend not a relative then it will come out by this type of examination. You can do it by filing petition before the concerned Magistrate for the permission to call Mr.X into witness box to adduce his evidence as it is necessary to prove the case.
Or otherwise you can ask in details in witness box with Petitioner/Wife; how Mr. X is related.
But you didn't mentioned the detail facts, why you want to prove that "X" is a family friend but not a relative, by this what help you can get or how it is going to help you in your case.