What happens if you work for cash and don't pay taxes?

My best friends brother works for a guy who is self employed and does construction type work. It's just her brother, the guy, and another person who take their tractors and equipment out to jobs and such. The guy just pays her brother cash, and they don't work steadily. Sometimes 1 week out of the month, sometimes 3 weeks out of a month. Here and there for about 6 years. But she's panicking because if he wants to apply for a "real" job with a "company" what will happen? Especially if he has this great experience and knowledge of equipment and stuff, how can he put that on a resume when he has not paid taxes for those years? (He's almost mid twenties now) Please any help would be great and I'll forward her the answers, or give her this link. Thanks!