The phone may need to be activated on that line because the phone needs to communicate with Verizon using the unlock code to prompt them to actually unlock it on their end. I recently unlocked one of my T-Mobile phones which I gave to a family member to use on AT&T, and this was the case. It's just another step they can perform over the phone using your info. If you can skype or Google Talk with your friend while they call Verizon, they can tell him any steps he must take with the phone to activate it on his line, then he'll message/tell you what to do. Then once activated, enter the unlock code and it should work. After it is successfully unlocked, he can go ahead and change back to his phone on his account by activating it the same way.
Kinda sucks to have to do this, but I think that is your best bet.
Droid 3 XT862 back on Verizon now, using custom build.prop. Can't seem to get ICS to flash to my Safe system, ugh!!!