Can first time be used as an adverb meaning "for the first time", e.g.
when I met him first time (Confession Tapes, third episode, 02:40)
"first-time" works as an adjective but usage as an adverb is quite unusual.
Absent more context it's hard to tell, but "When I met him first time" sounds like unremarkable casual speech. It's not technically valid syntax (being an elided form of "when I met him for the first time"), but it is easily understood and would not shock the listener.