You're going to be one kind of guy or another. Be the guy that follows the rules and expects everyone to follow them as well. There are ways not to come off as a jerk.
"Hey man, I was reading up on the barrel roll rules and it says right here **pulling out the rules reference** that if you lay your template to one side and it fits, then you have to play it since you've committed. And we can't measure if a ship is in arc until the Combat Phase."
"I want to be sure we're following the rules, so I'm not laughed at or kicked out of a tournament for playing wrong."
"Did you see the new FAQ? We've been doing a bunch of things wrong. Nym can only prevent one bomb from detonating and Genius makes you discard your bomb upgrade. And get this: if you use his ability to drop/launch after moving, it doesn't work if you overlap a ship, which includes if your Nym bumps another ship. That's considered 'overlapping a ship'." And of course print out the cards I posted earlier with the updated text.
Avoid saying things like "You" or it may come off aggressive and put him on the defensive. When you say "We" or "I" it kind of spreads the blame around and helps blunt the blow.
Also, include a third or fourth person in the conversation so they're both aware of the correct rulings and so he doesn't pull the same kind of behavior on them either. Chances are they feel he's played wrong too, but needed someone to say something first. There's safety in numbers after all, and there should be a consensus about how rules work.
Hopefully they're all agreeable and get back on track.
