I'd rather argue with him if Rey is a Mary Sue or not.
Rey's a Mary Sue because she's a scavenger who fended for herself for over a decade, became an experienced staff fighter, mechanic, and pilot prior to episode 7 as detailed by the plot setup and several scenes directing herself as such, and was able to beat a more experienced force user in a lightsaber fight because he was badly wounded and taking her lightly, but Luke wasn't a Gary Stu because a random farmer with junkyard swoop piloting skills totally translates to being an immediately excellent starfighter pilot, turret gunner, blaster marksman, and force user capable of directing a pair of proton torpedoes at long range with a single training session under his belt.
Luke isn't a Gary Stu because I didn't know what any of those tropes meant when I was 4!