In this situation, you're certainly not required to proactively tell your opponent that they can't draw a card. In other words, when they cast the Preordain, you don't need to remind them. When it starts resolving, you don't need to remind them. When they finish scrying, you don't need to remind them. Whether it's actually rude to tell them is a matter of opinion, not within the scope of the game rules, but personally I think most opponents who are not new to the game would be a teeny bit irritated at the reminder, because as players we're generally supposed to trust each other to act out the game correctly. (That being said, reasonable people would understand that you're just trying to make sure the game is played out appropriately, and would probably let it go if it's a one-time thing.)
After your opponent scrys, if they make a motion indicating they're about to draw a card, then you actually have a reason to think that they're about to violate the game rules. So if you do get a chance to interject at that point, it's probably a good idea to do so. But there's going to be a very short window of time where the opponent looks like they're about to draw a card but has not yet done so, so in practice it's probably unlikely that you'll get to do this.
Once your opponent has actually drawn the card they weren't supposed to draw, then it's a whole different story. That's a violation of the game rules and you must point it out if you catch it. From the tournament rules:
Players are responsible for:
- Calling attention to any rules or policy infraction they notice in their matches.