The Wii has the most variety. If he wants to try 1 or 2 games from 10 or 20 different genres, tell him to get a Wii. If he's all about third-person hack-and-slash or fighters, he should pick a different console, if he's all about FPS and RTS and online gaming and modding, he should go with PC, and if he's all about RPGs, he should pick a DS.
Wii dominates 2-D platformers, 3-D platformers, sports games, and party games, so it's the best system if you ever want to play games with girls. It's also great for drinking games. But the Wii library's strongest points are probably its variety, its several cheap niche games, and its ridiculous collection of classics on the Virtual Console.
Oh and if he's really so ultra hardcore that he thinks Mario's gay, he should get a Wii for the most badass super-manly ultra-violent murder simulators in the world. Wii has Manhunt 2, MadWorld, No More Heroes, and No More Heroes 2. Those games are so disgusting I have to play other Wii games like Muscle March and Captain Rainbow to calm down afterwards.