However, I'm leaning towards it being fictional, mostly because it feels similar to other creepypasta I've read (some of them are quite good, and highly recommended if you're looking for something unsettling). Of course, that idea could be implanted by the suggestion of it originating from 4chan, but there are some other things which make me lean towards that, especially the bit about misplacing/losing the game several years previously.
* Adding a "removed" state for pokemon in battle
* Adding the capability to playing sounds and BGM at modified pitch (or storing pre-modified copies of all sounds)
* Adding an "enemy is too scared to move" detection step in the opponent's battle sequence
* Creating the Curse move itself
* Creating a "tombstone" state for enemy trainers
* Adding a menu that appears after battles are over
* Keeping track of all pokemon species and trainers that have been "killed," in order
* Adding extra scenes beyond the end of the game
* Changing the way the battle engine deals with a battle where you have no Pokemon
* Adding an additional trainer-side sprite for the "old man"
* Adding a branch to the overworld music-loading algorithm that would always load the Lavender town music if you are in the endgame
* Adding a step to the map loader that removes all non-tombstoned NPCs if you are in the endgame
Anyone who could have done this at the time in question would likely be a professional Gameboy developer. It could be done more easily nowadays (especially using a Fire Red GBA rom instead) but back then this was complicated work.
* Change the saved game format to store and load all this new data.
That aspect alone throws the whole thing into doubt, since you would have to change hardware to support the enlarged save data. The original Red and Blue games already had unusually large amounts of save data for a GB game, enough that emulators at the time of their release did not properly support them, and I'm pretty sure it was well-compressed(within the constraint of guaranteeing a worst-case size). It's unlikely that it would fit after adding additional state to every enemy trainer and species in the game. (see: http://bulbapedia.bulbagarden.net/wiki/Pok%C3%A9mon_data_str... )
Definitely sounds fake, a poster above mentions several points that make it unlikely.
Oh wait, Nintendo wouldn't like that, would it?