Find a new friend!
Seriously, practicing eight ball is only one regimen I suggest to all players. With all those extra balls on the table, you must learn how to move the cue ball through them and which ball to shoot next. I believe your friend is incorrect. Practicing nine ball is good. However, what does it teach you about shot choice? You must shoot the next numbered ball. Every game you practice might teach you something. I try to play three cushion billiards. I'm not very good at it but, I don't jump balls. I need to be able to hit my ball when snookered. What do you do? I love bank pool. Again not very good at it. Learning to make basic banks helps me both in the safety game and making the ball when possible. I love it when my opponent pushes out to a high percentage bank. Although few play it, 14.1 is a great game to learn. Position, position, position! You don't have to be world champ, just learn the basics. Same with one pocket. Although you may not shoot at your pocket on a regular basis, it will teach you patience. And that is one of the greatest weapons of all!
Lyn