You have to understand how a ball's spin affects its flight path. For a right handed golfer, a clockwise (when viewed from above) spin causes the ball to curve to the right (slice). A counter clockwise spin causes the ball to curve to the left (hook). (Of course the primary rotation of every shot is backspin. But on a slice the axis of backspin rotation is tipped to the right; on a hook it is tipped to the left).
A bad slice is usually the result of two components: An outside in swing path coupled with an open clubface at impact. This causes the club face to "scrape" across the surface of the ball at an angle making it spin clockwise. A severe hook results from a closed club face at impact together with the club approaching the ball from the inside. Of the two components, the club face angle affects ball flight more than approach angle. But the two together cause the most extreme ball curving.
An inside out swing with an open face causes a straight (no curve) push to the right. An outside in swing with a closed face causes a straight pull to the left.
The ideal flight path is a draw, which is produced by slight, controlable counter clockwise spin. To accomplish this, you have to have the club face square (perpendicular, not closed or open) to your target line at impact and have the club face approach the ball slightly from the inside. This imparts a slight counter clockwise spin. The ball will start a little to the right, then curve back to the center. A fade is the opposite of a draw.
Swinging as if you were trying to hit a baseball to right field produces the inside out swing path. Starting out by over exaggerating a corrective motion can be helpful. So, your friend is right that intentionally hitting severe hooks may help eliminate the slice. But what he didn't tell you is that to hit the "duck hook," you have to swing to right field with a closed club face. This can be counterintuitive, but to make the ball curve left, you have to swing to the right and vice versa. It is sort of like having to hit down on the ball with your irons to make it go up.
