The reflection is only bidirectional if you can see the other person's eyes.
It's like if someone is in a bathroom stall. You could see the stall is occupied by seeing their feet stick below the wall of the stall, but they cannot necessarily see any part of you since their eyes are not where their feet are.
Same principle applies to reflections, where maybe the body part that you can see is just the top of the head, and since the person isn't tall enough they can't see that you can see them.
You can see other people in a reflection without them being able to see you.
It is not possible to see someone else's eyes (except from the side, so it's only seeing their eyes in profile) in a reflection without them being able to see you too.
It's literally not possible via reflection, as everything is equal and opposite. If light can go from their eyes to yours, it's also possible to go the opposite direction.
This is what everyone has been saying but instead of thinking through everything clearly, you resorted to bullying.
The only way to accomplish this one-way vision is by adding something that is not reflection to the system (like a one-way window), but that's breaking the premise under which everyone else has been commenting in good faith.