First to answer your question: I think the eye of a snake is covered by a layer of film, or the so called lense. It works just like its scale, to protect what's underneath, in this case the eyeball. So when snake sheds the eyecap area also comes off.

If during a bad shed the eyecap part did not come off, raising the humidity or soaking the snake in the water may help. In your friend's case I really have not heard off; but since the layer of film covering the eye works as a scale, I reckon it will grow back after a few more sheds. Hope this helps.