I Believe It Is Illegal Window Tint

I had a buddy of mine recently he just had his windows tinted on his brand-new Ford Mustang, and I have to tell you it made his car looks so much cooler than it already was. On the benefit side, he was protecting his interior from the sun's rays, but on the other hand, I believe it is illegal window tint. But for his sake, I certainly hope it is not.