It would take a lot of time and effort. You would need to create models for the pokemon. Even with the damage data textures, there would be too many and it would end up lagging the game. The other problem is having to make it multiplayer compatible, including teams and battling the teams. It would be difficult to create how pokestops appear, where gyms are, ect. If he was planning to do this, it would take months. Not impossible though. You just need to find out a way around the lagging part. Plus, there already is a pokemon mod.