If you let him have the actual copy, when you go to fight the Griffin at Blue Moon Tower he'll run up with the book and one shot the bird. If you give him a fake he'll run up and try but fail miserably. I'd recommend not giving him the book because that moment lost a lot of the epicness by him just one shotting it.
"Not one of the 12+ people killed were armed. They were all civilians."-beerconnoisseur The above quote made me die of laughter. I think I'm evil...
You still get exp, and the meteor spell (I believe it's called bolide) can be learned by the sorcerer vocation. So really I'd just recommend giving him the real grimoire, I don't think it's worth forging it.
Just as a note, he's SUPPOSED to one-shot the gryphon, but in my game, he ran in, did his little grimoire thing and I'm pretty sure the meteors almost missed the gryphon entirely, so I had to kill it myself anyway.