First things first, have an idea where you want the Slider to end up. That all begins with a Down Hill Mindset.
Know that the ball is going to break down and away from the hitter (if righty vs righty or LHP VS LHH).
The best Sliders start out a strike and end up a ball out of the zone.
Throughout the delivery, think Fastball, Fastball, Fastball...SLIDER. This will help you stay behind the baseball and release it out in front.
Another tip to consider, is not trying to throw the Slider in the arm swing out of the glove. When pitchers speed their arm up at this point, trying to throw the heck out of the Slider, the hips open Too soon, the shoulders open up too soon, and the hand fall underneath the baseball. The Slider loses its tilt when the foundation of the delivery goes to waste.
Stay under control and focus on getting the arm up on top, then pick of the arm speed going forward.
Best of Luck with the Slider to you and your friend!!!