Lore with the game, is what I believe Kris is referring to. Lore with the movies have never correlated well with Disney's intentions in-game. However, I entirely disagree. Guns being unable to damage human opponents whatsoever is a downside to them, and what holds them back from being more versatile than practically any other weapon class.
Secondarily, the game is still (to my knowledge) to abide by the original ratings given to POTCO, which would surprisingly put it listed as a game for younger players. It may not be like that today, but there are some things that can't be changed because they feel like it.
I understand that your friend has the intent to play through the game with guns, but POTCO was made with using the entire arsenal in mind, and limiting yourself to one weapon might help you min-max loot or whatnot, but it takes away from the overall experience.
Fair Winds,
Misha