I just tried this and its not straight forward if the verizon friend already has an iphone device and plan from verizon. The phone comes preactivated with the existing number of verizon account. The moment the iphone is turned on it registers the new iphone and disables the existing one of your friend. I'm not comfortable doing this nor would my friend like it if it has a risk of his number getting deactivated.
There is a workaround to this. When you get the phone, take out the SIM card that came with the phone before turning it on and activating it. Then just put in your existing SIM card and switch on /activate with that. It works and nothing will happen to your friend's line. As an alternative, you can just take the phone to a Verizon store after you receive it and they can dissociate it from your friend's account before activation.

