I don't totally agree with this. But there is a fine line. I'm friends with both of my kids. But they also know we have expectations, rules, and family values to live by. It's more about being active parents in their lives.
I don't think anyone can generalize that there is any single right approach at parenting for every kid. Undoubtedly some people take the "be the friend" approach too far, just as people have always take the "firm disciplinarian" approach too far to the point of actual abuse.
My own two kids are completely opposite when it comes to punishment for example. My older son responds very well to time outs for behavior correction, as he stops, thinks about things, and feels guilty. It doesn't have the same effect with our younger son at all who needs more than that.