As far as I know there is no special combination of buttons for a factory reset. Tell him to try the telnet method below failing that just flash it using the latest dreamup.
Log into box via telnet.
mount -o remount,rw /boot
rm -f /boot/*
Open up a web browser and type in the ip address of your dreambox (which will be one it picks up via DHCP not the one you had set in the config). This should take you to a web page. Choose the upgrade option and point to where you have an image stored.
Before you do this you must:
Make sure you have a reasonable way to pick up the Dreambox via DHCP once you've nuked the image.
Have an NFI image to flash
Have a machine to connect to the Dreambox web interface with that can see the Dreambox over the network (this is where you'll flash from)