He won't see the screen as the BIOS will attempt to show up on the laptop screen even if you hit the correct BIOS button.
I suggested this in another post to another user. Get an external mouse and keyboard, turn on the laptop, close the case right after that. If it does not go to sleep immediately, it should boot up and see the external monitor as the main one, and send the BIOS display to that.
If that does not work, you may need to get a docking station for your model, they usually give you a power button on the dock so you can turn on the laptop without even opening it. Then for sure the laptop will only output to the external monitor. I have seen used docks on ebay for as low as $20 for some systems.