Did he switch from onboard video to the ATI card? Is the ATI card AGP? Did he seat the video card into the slot properly? When you say it doesn't boot up, do you hear a series of beeps when you turn it on and then it hangs or does it boot up but there's no video?
As you can see, there's a myriad of reasons as why it won't boot up so that's why you have to be a little more specific with the problem.
1. If he switched from onboard video to the ATI card, make sure that the onboard video is disabled in the computer BIOS.
2. If his old card was a PCI video card, then again, in the computer BIOS, make sure that the primary display is set to AGP (make sure that the AGP speed is set to whatever is compatible with the video card, should be either 2x or 4x for that particular ATI card).
If you hear a series of beeps on startup, remember the series of beeps and look in your motherboard manual and it should tell you what the series of beeps meant.
If the computer boots up but there's no video, make sure that the monitor is connected to the correct port and follow steps #1 and #2.
Ack-Ack