checking it from the mobo is better since some cards have both agp and pci-e or pci versions. But seriously, just open the case and look. How could he even brake anything when just opening the case?
But eventually he will need to open the case anyway if he wants to put the vid card in.
And also tell him to check that the psu has enough power to run the new vid card. And u probably want to open the case when checking for the psu wattage.