If he goes from "Pictures" (the Vista equivalent of "My Pictures" in XP) and highlights the pictures he wishes to burn then selects "burn" from the tool bar at the top the dialogue box comes up saying insert disc. It will burn a CD from there.
In the Windows programme "Windows Photo Gallery" if he tries the same it will select a DVD.
The most likely explanation is the preferences for the drive and programme have been set to DVD particularly if he has a new machine.
It is also possible he has a third party photo editing software with the machine which is set up for DVD.