TimO, I don't see anything wrong with your stance. From your description, it sounds like you have more of a closed stance. I actually think more pros are closer to your stance than the neutral stance described by your wife's coach. McEnroe, Federer, Cilic and Sampras are just a few who use more closed stance.
However, you could learn to use the neutral stance and then rotate hips and shoulders to close your upper body to the net. Kind of like Nadal.
But, I think it is more preference than anything. The coach kind of showed you the basic 101 serving stance but there is nothing technically wrong with starting more closed rather than rotating the upper body. I am probably in between - slightly more closed than the coaches neutral stance and slightly more open than your stance.