I'm of the opposite opinion here. If the guy KNOWS which foot forward he is, as in he's boarded a few times on rental boards and one way is significantly easier for him, then I'd set up the board more directional favouring that foot forward. Something like -6/+18, -3/+18 or so. It'll make the basics easier while he's learning how to turn and hopefully carve one day.
Duck stance may make people favour heel-sliding everything instead of forcing them to learn to turn.
My GF has learned from scratch in the last couple years. She started with a stance above, then we moved them 3 degs towards the back to something like -9/+15, and then last year we moved it to -12/+12 and she's happy.
Stance width is important but as long as the board is suited to his weight, and he's not excessively stocky or lanky, then reference stance width will be fine to start.