The + is used for the country code. He probably has friends overseas who has an iPhone.
Not necessarily.
If you enter all (and I mean all) of your phone numbers as international numbers (ie + then country code then area code then number) then they will work no matter where you are in the world. So if you travel a lot it saves you having to edit your phone numbers before calling them.
I do it even with all of my UK phone numbers, they just dial as normal when I'm in the UK and abroad. You don't get charged anything extra for dialling them as international numbers.