How far wide your feet are is totally down to personal preference. Wide and narrow stances both have their pro's and con's. A wider stance is more stable as it forces you to bend your knees more which lowers your centre of mass, BUT, it is generally harder to be subtle when trimming the board's height - it's a bit all or nothing because you have to move your body/hips further to shift your weight forwards and back. It does give a much more secure feeling when cranking turns though.
A narrow stance just feels more relaxed and chilled. It's easier to make the subtle microadjustments for trimming pitch as you don't have to change your body position as much to alter your center of mass. If you listen to the Progression Project, quite a few of the top foil surfers admit that they actively try to develop a narrow stance partly because they like the feel of it, but also because its a style thing - they prefer the look of it. My take on it is to go with what feels right for you. Check out Keahi's foot positions on the video. He constantly changes it depending on what he's doing. For turns his stance is always wider. When winging, whenever I go for a real quick gybe to turn onto approaching swell I always widen my stance slightly and bend my knees more than usual. I reckon it's more important to have a wider stance for foil surfing (whether prone or SUP) than in winging as you can use the wing to support yourself to a certain extent.