@poxeveryturn The preordain was to make clear that Player A went past combat. Intuitively, the two cards should cancel out and since this is a Vintage event, I doubt a judge was called. However, the rules are different and part of this was to inform players how this interaction works. You might want to mention to your friend that this interaction is more complicated. He or she might appreciate that if it comes up again - In general the board state is much more advantageous for the Oath pilot since Ballista doesn't do damage, Foundry Inspector deals 1, etc.
Actually, this leads to another question... If the opponent casts Metamorph on Gisela and then sacs all their artifacts to Ravager, then modulates onto Walking Ballista, then says "Shoot you for Lethal", does that mean their Ballista goes to the yard and no damage is dealt?