Without knowing what he has installed it's hard to say.
This that do use data and is known.
Mail, Contacts, and Notes if you have it to sync without iTunes(using iCloud services)
ANY app with advertisements, or apps with DRM checkers(DRM check uses a very small amount to actually count)
downloading apps not on wifi
Apps like Words with Friends, Scrabble with Friends, or any app that connects to the internet to play(even Multiplayer games unless it uses bluetooth to connect).
Maps(it caches a 5 mile radius, but if you go beyond that 5 miles it downloads the next 5 miles and deletes the old one IIRC)
There may be other stuff I don't remember off the top of my head.
4 one thing is your friend using the IPhone on the Web ( Using Safari, Opera, and/or Skyfire)? If so, that will cause your Data to go up really quickly. I have had my 4G LTE 551L USB Modem only for a couple of days now, and it is all ready almost at 5GB Usage. Lucky for me I went with the 10 GB Plan instead of the 5 GB Plan! :-)
Message was edited by: KEVIN <personal infornation deleted per TOS>
Message was edited by: Verizon Moderator
streaming music or videos (pandora, youtube, etc) will eat it up quick. if you play stuff that's in your memory (items already in your itunes) then it doesn't, but if you "stream" them from apps or sites, then data can be used quickly. it does start over every month. your best bet is if you have internet at home, then get a router and hook up a wifi network. whenever at home, use wifi (or really, anywhere wifi is available. at work, for example).