Lol by taking his doubles you ARE screwing him over

When you tell him when to courtesy shuffle you're basically handing him a +EV game, assuming he has his basic strategy down, and he can still play his progressions. In that case you could accept some form of tip and he could still have +EV depending on the size. But even if he does overtip you, is gifting money to a friend the same as being screwed over? Would that very same money go to the casino instead? Wouldn't you take a cut of what the casino gets through your AP anyway?