Um, you said you're,
"Trying to delete a wallpaper not files."
ALL wallpapers ARE files.
They are bitmaps in the (normally) windows folder.
All the registry does is point to the location of the bitmap.(and possibly lock it.)
I successfully used the tool I linked you to, to delete a running wallpaper on my own machine, and it's very easy to use.
If the system is clean like you say, all you have is a locked file, have him kill it with the tool and reset a new wallpaper.
You asked for an unlocker.
Why ask for tools if you won't even attempt to use them? d/l it, run it and tell your friend what he needs to do, if he/she can't right-click a mouse they don't need your remote help, they need a shop and a tech.
I understand you are trying to help an un-savvy friend, but you're not likely to find a 1-click "Magic Bullet".
ED