A boy and a girl who are conducting a haraam relationship asked me to perform their Nikah, but no one should be informed.

Answer

Secret nikahs are extremely problematic. It is not proper to perform a nikah when the girl's parents are unaware or they refuse to consent. Advise the couple to be brave and to speak to their parents and to accept their advice. One should not embroil oneself in something which will surely erupt in fitnah. Zina is a secret affair, and Nikah is a public affair.