Hi yes you can sure give your iphone 3gs to your friend but he can't use it with vodafone sim until he unlock it from o2 ...If you want to unlock your iphone 3gs from o2 .Then have a look at sites like Mobile-Unlocker.com here they will unlock all model iphone using remote unlocking service for affordable cost .After unlocking the iphone you can freely use it with vodafone or any other GSM sim .
