Tell him about hell... always works.

[Uhh, no.]

Yes, and it doesn't matter if he believes in salvation by works or not. If he does, he will stop sinning, and if he doesn't, he will know that his faith is as for currently dead faith as it is called in the gospel.

Of course, he needs to know Jesus before that. If he does, he will repent and if he doesn't... well then you can easily convince him of that he isn't a christian.

Speak a lot of hell and eternal punnishment. Especially when the "christian" is experiencing pain, like during a intense gymnastic lesson or when struggling with an essay.

[That's a great way to alienate your friends. :D]

If they are oversensitive, here in Sweden, most are not. (although you could very well get a club in the head for that. :D)