Using an Ultrasonic Sensor to trigger on a Screen in a Car

So someone I know who has limited understanding of Arduinos, asked me about a possible project for a client of his.

He wants a system to trigger on a display(iPad or some Android tablet) in a car when the passenger is seated in the back seat. The display will be placed on the back sides of either one of the forward seats. He was planing of using a dome Infrared sensor to detect the passenger. I suggested he plan of using a Ultrasonic sensor instead of the Infrared sensor.

I would just like some suggestions on how to go about it and what all will be needed to go about doing this. Apart from the arudino, display and sensor. Thank you.

Seems a bit extreme when all one has to do is lean forward and push the "on" button.