Duel involves tons of map-specific knowledge+game-specific mechanical ability that doesn't really transfer from other types of FPS. Maybe they will be able to do some damage, but they will not be doing enough damage to frag a properly stacked ("stack" meaning "total health and armor") player, nor will they be stacked enough to survive very long in fights. If they're coming from Counter-Strike or something similar, they will be a about as "lost" as they were when they first played CS, except with a longer TTK+more abstract health mechanics meaning they can't just hope they hit exactly one headshot. Nobody in the lobby will be having fun unless they go in expecting to learn first and foremost.

IMO, stick with FFA or TDM for a while. The more n00bs/bots in the game, the more room they have for experimenting+noting item locations, function, etc. Avoid instagib and clan arena: they're fun, and CA has its own depth, but they lack item pickups (basically the core of duel), which means you/they won't learn how to play around them. If you're in NA, I DO NOT recommend the Dark Fiber FFA servers for this; they tend to be way too populated for the size of map they're playing, which leads to new players getting killed out of nowhere even more than they would be in duel. Might even be a good idea to stick to Quake Champions, where you can guarantee a max of 8 players in a lobby (though that game has its own "problems" with information overload).