If his Windows XP installation disc was built by the manufacturer (i.e., has Dell's logo in addition to Microsoft's), it will validate the BIOS as being from the correct OEM before permitting installation.
He would have to purchase the motherboard from Dell (if it is still available) or else have access to their BIOS tattooing kit (usually only certified techs get that).
And as for using a different CD with his key----no. Windows XP discs supported different sets of product keys depending on their distribution channel. A Dell OEM key will not work for a full retail, volume license, or retail upgrade disc. His Dell key will only work with Dell media.
Microsoft knocked off that nonsense in Vista, thankfully, but shops with legacy systems will have problems until the day XP is no more.