"full photographer experience" is the kicker here really.
Can you do weddings? Apparently, because your friend does.
Can you create a dramatically lit portrait in a fully lit, bright room against a bright walls without it...probably not.
If you always have control over your environment and only want to do certain kinds of shots - then of course you can do it....but you're certainly missing out on a whole dimension of your photography you can control.
Shooting without flash is part of his style and thats what he's selling. There's not really an inherent advantage to refusing to use flash - its just that its helping to define his style. And having a style - that's an advantage.