You're probably right about it being rude to say that to a friend. That being said, you'd have to be frank with your friend that (if the restriction against drow ninjas is absolute) he likely wouldn't have much fun in your game with his niche gone.
I try not to put my friends in situations where they're not going to enjoy themselves, so I'd steer my friend away from my game if there was no way to accomodate his niche. On the other hand I might deribe the fun another player is having paying something similar (such as a halfing sorcerer focusing on stealth magic and illusion).
I don't know. It's really hard to call, sometimes.