Your friend is kind of correct, in that he's right if you don't include vehicles.
For each To Wound roll of a 6, the target automatically suffers a Wound, regardless of its Toughness. These Wounds are resolved at AP2.
So, it bypasses toughness, not saves. But, they are resolved at AP2, which means you're relying on an invulnerable save (or cover save), since no armor is better than 2+.
But the distinction comes into play when shooting at vehicles:
In either case, against vehicles, each armour penetration roll of 6 allows a further D3 to be rolled, with the result added to the total. These hits are not resolved at AP2, but are instead resolved using the model/weapon’s AP value.
So a vehicle with a 3+ save would still get a save against rending AP4+, whereas an infantry model like a terminator would rely on his invlun save because of AP2.
EDIT: This is all in the big rule book, but you won't find it in your codex.