|Not enough information. Post all specs on original system and the changes.|
If he is using a video card and the new motherboard includes onboard video capability, he may need to plug the monitor into that until he can get into the BIOS set up and adjust the settings to use the separate video card instead.
He also may need to clear the CMOS (power unplugged) before trying again.
Did he plug in ALL required power plugs? (24pin connector, 4 or 8 pin connector, etc.)
You have to be a little bit crazy to keep you from going insane.