I want to get my friend into FO:NV and I believe the best way would for him to play a modded version rather than vanilla, however he disagrees. He has just finished Fallout 3 for the first time and said he said it would be in his top 25 games of all time. He also played it on the Xbox 360, so no mods. He has never played New Vegas and while I didn't play a modded version the first time I think I would of wanted to. I didn't find out about mods really until after I already played the game. Since I found out about them I've never played it without it being modded heavily. Most of the mods simply make everything better. Graphics, bugs, performance, physics, weather, armor, guns, ballistics, etc. He says that he wants to see the game 100% as intended by the developer.

What do you think, should he play with or without mods?

Except for bugfixes/patches I think you should always play a game as vanilla as possible for the first time through. Or at least a few solid hours in.

This way you get to see the game as the devs designed it. And how do you know what you want to change unless you've had hands-on experience with the game?

But really, it's up to the player's preference to install mods or not. There is no wrong way to play, so you should respect his opinion to see vanilla