If its a prebuilt computer he has such as a Packard Bell, Dell or Emachine etc it will tell you on the back the make and model number. Then you can go to the manufacturers website and look it up.
Alternatively you can look at the motherboard manual if there is one.
Or you can try either SIW or Belarc both are free programs and will tell you all about the PC. If using one of them you need the name and model of the motherboard. IE ASUS A8N-SLI or Gigabyte GA7VAXP or whatever it is. Then goto the motherboard manufacturers website and look it up.
SIW click here Belarc click here
One thing to mention do not confuse the new PCI Express slots with the older PCI slots as they are completely different. If the computer is over say 2 years old it may have AGP but will not have PCI Express.