Where you getting a display on the screen even though it was cracked? And you're talking about a physical crack not lines of colors going across the screen without any damage to the LCD?
If the screen was just cracked and you had an image, it's not the motherboard. Or at least was not the motherboard when you brought it in, could have broken while he was working on it. Video card and/or motherboard issues will show as funny colors or no display, nothing happening when you try to turn on the laptop, things like that. You can try to run the laptop with an external monitor, if you get a display on that, it's not the motherboard.