a (person) (someone) knows

"An acquaintance" is a person you know who's not close enough to be called a "friend". But the word "acquaintance" is pretty formal. When you're telling a story or talking to a friend, it's more natural to say "a person I know", "a woman he knows", "this guy she knew", etc.:

This guy I know is from Laos, and he said that there are a lot of expats living there now.

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