My friend asked me about a person I never met in my life.
Should I say "I have never met him in my life." or "I never met him in my life."?
In this case, the past and the present perfect express basically the same thing, so semantically it doesn't really matter which you use.
Other things being equal, answers very often reflect the form of the question. For instance:
SHE: Surely you met him last August?
YOU: No, I never met him in my life. ... your past tense matches hers
SHE: You've met him, haven't you?
YOU: No, I've never met him in my life. ... your present perfect matches hers
This is not by any means a rule; but it may help you in conversation.