If you are permanantly handing over a harddrive (whether in a computer or not), you should ALWAYS do a security wipe. The reason is simple. You simply don't know who will have access to it over the life of the drive and "undelete" and "restoration" software is easily gotten. Financial information, passwords, social information, pictures, chat logs....even porn...there are too many things that can be left over lying dorment that can come back to bite you.
You trust your friend, but when it comes to this sort of thing you shouldn't trust anyone. It's about privacy, not law.