1) The Cowboys only won one Super Bowl under Coach Switzer. It was an extremely talented team. There are a number of coaches who could have led them to a championship. That said, if your friend from Texas said Jimmy Johnson deserved all the credit, I wouldn't argue. If he says Jerry Jones deserves all the credit, he's an idiot.
2) Off the top of my head, I don't remember any players going to prison while at OU under coach Switzer. Jerry Parks, Nigel Clay, Bernard Hall, and Charles Thompson all committed acts that eventually led to their incarceration in early '89. Coach Switzer resigned shortly after Thompson was arrested. I'll be the first to admit that things were out of control at the end of Coach Switzer's reign in Norman. However, that's 4 guys committing acts in about a 2 month period. It's not as if OU had guys going to prison every year. Some of Switzer's players, the late David Overstreet comes to mind, ran afoul of the law after they left OU, but that happens at most programs. For example, Leonard Little and Dwayne Goodrich have run into serious legal troubles. I don't hold Fulmer responsible for them. Sounds like your friend was probably a Texas fan.