I would first suggest that if you need help upgrading a CPU then you aren't best qualified to help your friend (you may be doing him more of a favour by having it done by a technician)
Anyway, onto the answer.
There's a 75% chance it is socket 423. Most of the slower - i.e. 1.4-1.6ghz P4s are socket 423, which is now obsolete. You won't find a new CPU for that motherboard anywhere. You may be able to find one on eBay, but I'd almost guarantee there won't be any new ones.