Answer:
The device will probably overheat. *
Step-by-step explanation:
The continual input of power will cause the laptop to heat up, and later, when he unplugs it, it will cool down. This cycle of warming and cooling will probably damage the battery as well as other parts.
* however, this being said, most devices these days have firmware to stop them from overcharging, plus batteries have improved greatly, so there is probably not too much harm in leaving a laptop plugged in