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.
It was one of those PCs based on Windows ME and Rambus memory of the early days of Intel's Netcrap architecture. Best solution is to scrap that motherboard altogether and get something more decent and recent (e.g. LGA775 or AM2 CPU, motherboard and DDR2 RAM).
I sure hope your friend isn't using Windows ME anymore. Microsoft has finally stopped supporting this piece of s***!