Does your iTunes account have contacts attached to it (from your personal outlook/contacts account)? If it doesn't then all will be ok, you'll just get a list of his contacts on your pc, but if it does it'll synch your contacts to his phone and his contacts to your contacts list which will be a nightmare figuring out what's what.
When you sync you can also choose if contacts are synched and which account they're synched to (ie. outlook contacts, googlemail contacts, yahoomail contacts etc) but you have to make sure that "auto synch iPhone when connected" isn't set. I don't think that option appears unless you connect the iPhone first though, which is annoying.
If he's on 02 they have something called a bluebook I think it is. Have a look on the 02 website and see what that's about as I think that's a way of storing his contacts. Then he can just go online and pick them up from there or you'll be able to export the details across to him.
I hope that makes sense and helps? Let me know if you need any parts of my whitterings translated though.