### Simply forbid it. The specific reason why is just fluff when your concern is strictly mechanical

In every TTRPG I run where a situation like this even *seems* like it might come up, I remind my players that I have do not have any responsibility to break the game nor to assist them in breaking the game. Quite the contrary-- the DM has an explicit responsibility to keep the game challenging and interesting. If the players boost their CON scores to 30 then many aspects of the game become trivial, and so executing this plan is the conclusion of the campaign. And not an exciting one. **One of your primary tasks as DM is to make sure this does not happen.**

They have no technical argument to fall back on, as your time dilation is already homebrewed and takes everyone beyond the published rules. If your group has already become fixated on this exploitative plan then any obstacles to it are likely to feel cheap, *unless* they appreciate your responsibility to the game. The main impact of this is that it is probably not worth your time to try and whip up a completely satisfying, in-universe reason to limit them as the only reason their idea won't happen. It won't matter if someone in the Feywild steals the book, depriving the party of it forever, or if it's destroyed by erosion or natural disaster, or if you decide that the manual is cursed, or has some other properties which interfere with their current expectations, or if you alter the game's challenges such that their arbitrarily high CON scores are irrelevant.

My experience as DM is that players are often a bit deflated at seeing their cool idea not come to fruition, but ultimately accept that it would break the game to do otherwise. Just last week I allowed my players to choose one magic item each, very rare and lower, from a wealthy genie as a quest reward. I reminded them that their choices were subject to my approval. One of the players wanted *Illusionist's Bracers*, but the way he'd specced his character out meant that they would be wildly overpowered and so I did not approve them by saying that the genie didn't have such an item in his collection. He was disappointed, but understood.

**tl;dr: Abusing the manual will very likely ruin the game, which you as DM cannot allow. Making this clear is the only way to avoid such abuse without making the players feel cheated now that they've already conceived of this plan.**