I'd throw out there that he can probably commission an artist to do a book cover and use that as a graphic for the Facebook Cover. If he's not rolling in the money, however, he could still do this, but target student artists or those trying to build their portfolios. That would allow him to get a book cover for reasonably little expense, while still getting a quality image -- plus he'd help another artist just trying to make it!

That said, if that's a path he's thinking of going I would be willing to talk with him. I'd be happy to produce a little work and hear about his ideas to create a cover, plus a bit of imagery to use as he sees fit.