In Civilization V can city states steal workers? I've played Civilization V for over 600 hours and I've never had a worker stolen by a city state. Playing a hot seat multiplayer game when I went AFK my friend said Brussels stole one of his workers. Unfortunately a barbarian camp was right there so I'm not trying to say it's not possible but he is adamant that it was the city state and not the barbarians.
Straight from the Wikia page about City States:
...This changes, however, if the City-State is dragged as an ally into a serious war between civilizations. In this case, the City-State army will not only defend, but also attempt to attack nearby hostile countries (usually despite being vastly outnumbered). If the enemy, be it a nation or another City-State, has a city right nearby, the City-State will send their units to attack that city. Their armies can also pillage improvements and capture Workers. In very rare cases they manage to capture another city, which is then added to their own territory (but their expansion stops)...
There is no reliable information to suggest that a city state would steal workers if they were at peace with that given player.
So it can only presumed that my friend misinterpreted the barbarian unit that stole his workers as being from the nearby city state.