In my case, my buddy knows his tendencies and generally avoids mixing player knowledge with character knowledge. He likes to have fun, so it’s a knowing suspension of disbelief thing for him.

It’s more about messing with him and giving the group a good, unique experience. “Oh shit, this hill giant can cast misty step?” That kind of thing. Or adding damage resistance or swapping abilities or increasing crit range or juicing hit points. Keeps the knowledge gamers on their toes and makes it more fun.