You mean re-cut the slots? I'm not experienced builder, but that doesn't sound good.
Also, the length is already established, which is part of the problem.
Also, if it's a purchased pre-slotted fretboard, that indicates that he might not have quick and easy access to a new piece of fingerboard wood.
As for the scarf joint, a photo is worth a thousand words-- I bet if you do a forum search, you'll find one. I have photos in my Project Lucy thread, page 4, but they're for a new neck blank and don't account for a shaped neck.
I certainly wouldn't want to be doing a scarf joint on a shaped neck-- not that it can't be done, but I don't trust my own skill level at the moment. Also, with the guitar being neck-through, it'll be hard to line up the wood for an accurate cut.
Not that it couldn't be done; I'm simply saying that I personally wouldn't want to be the one doing it.
Greg