Yes, under the GCA68 it is unlawful for someone who holds an FFL to keep another's firearm while it is being worked on UNLESS it is logged in that person's bound book. However if it is done while they wait, no recording is required.
Nothing is said about one friend loaning a gun to another friend who just happens to repair or modify it while they are borrowing it.
Neither is anything said about taking a gun to a friend who does repairs for a hobby.
Shipping a gun to anyone other than yourself, requires at least one FFL to be involved in the process.
In the case of the aformentioned builder's class in order to ship it away to be blued/inspected the owner must ship it to an FFL holder who then loggs each gun in their bound book. After it has been inspected/blued the FFL holder is required to ship it back to the original sender.
For example, if you buy a firearm from Joe's Gun Shop and it breaks and you then take it to Fred's Gun Shop to have them send it back for warranty repair then the repair station MUST send it back to Fred's Gun Shop.*
However if you had sent it directly to the repair station then they could lawfully ship it directly back to you.
*The exception would be if Fred's no longer held a valid FFL. Then a substitute FFL could be designated.
Ain't Federal law special?