I don't know if an external monitor kicks in that quickly, has he been able to view it at any time before windows it up and running ?
But the Esc key should work to stop the scan, if it isn't stopped the scan could be quite long on a low powered laptop if it has a lot of data on it. However 4 hours is a very long time.
Setting it to a boot-time scan should only be a one shot deal, e.g. it should run once and not again unless he schedules it again, in which case it would run on the next boot.
So there would appear to be something else going on here.
What is his Operating System ?
What version of avast is he using including the full version number, avast free 5.1.889, etc. ?