Read adsense TOS, but I definatly don't recommend it, specially because blackhat SEO is pretty much dead these days. [now this depends on what you consider blackhat]
What blackhat technique exactly? It depends on exactly what is he doing. Otherwise nobody could say if AdSense is going to ban him. But Google might decide to ban his site.
If you mean repeating keywords over and over it is pointless as the best way to gain traffic is by decent honest hard worked backlinks and quality content that people come back to see again and again.