I read that changing a motherboard is now considered as a being a new
computer by Microsoft and requires a new licence.
Not correct.
First of all, note that a new computer requires a new licence *only*
if the license is an OEM one. There is no such restriction on retail
licenses.
Second, even with OEM licenses, a new motherboard does not mean it's
considered a new computer. For a long time this was unclear, but
Microsoft has now clarified this. See
http://www.microsoft.com/windows/products/windowsvista/buyorupgrade/a...
or
http://tinyurl.com/384gx5
which states
"If you acquired Windows Vista pre-installed on a computer from a
major manufacturer (sometimes referred to as an Original Equipment
Manufacturer or OEM), Windows Vista will require re-activation if you
replace the motherboard with a motherboard not provided by the OEM."
That's about Vista, but the same would clearly apply to XP as well.
This doesn't seem very fair if the motherboard has become faulty and needs
replacing. Will he have to get a new licence if I change his motherboard?
It will be a second hand one and not new.
However, bear in mind that if the computer is one from a major OEM
(such as Dell), the copy of XP is very likely BIOS locked to the
original motherboard. Unless it's replaced with an identical one, the
old copy of Windows will no longer work on it.