The bigger the bottle, the smaller the ratio between exchange surfaces/volume. Small ratio means slower reactions, longer conservation, which is empirically better. And you know, a big bottle, so you don’t have to leave the table.

My friend the professional sommelier says a magnum is the perfect size to share with a friend for lunch. Especially if the friend is driving so they can't drink too much.

I think this is probably not right. If you always have exactly one bottle open, the magnum will always have more total surface area. If you drink less than 750 ml from both, yes the ratio of area/volume will be smaller, but conversely it also means you are tainting more wine. And if you wanted that effect, you could always open the second bottle the next day and mix the two regular sized bottles.

Your right - where this comes most in to play is that the wine isn’t stationary. Every time you pour, which will happen more often with a magnum, you encourage interaction between the wine and atmosphere.