Rojopero,
Hopefully your relative purchased wisely and got a genuine OEM license kit.
Since the computer has XP Pro installed now, you will be able to attempt to use the Product Key Updater utility to "genuine-ize" the nongenuine installation of XP Pro. The Updater will take the PK from the OEM license's CoA and try to genuine-ize the current installation of XP.
Here is the link to the utility: http://www.microsoft.com/genuine/selfhelp/pkuinstructions.aspx
Please note that the Utility is not 100% effective----there are situations where it will not be able to genuine-ize a nongenuine installation of XP, and if that happens in this case, you'll have to backup and offload anything that you want to save before you do a clean installation of XP Pro using the OEM license kit. Here is a good tutorial for doing a clean installation: http://www.michaelstevenstech.com/cleanxpinstall.html
Please post back to tell us how you are doing!