Your friend is completely wrong. As you know, there is only one BCM in a 4th generation f-body. In addition, the BCM has absolutely nothing to do with the radio other than providing the retained accessory power (keeps the radio on after turning off the ignition until a door is opened).
Newer GM models that come with double DIN as standard equipment have various other system functions embedded in the head unit such as the alarm, warning chimes, and sometimes even the heating and A/C control. But our cars have none of that.
Do a search in this forum for "double DIN" and you will get plenty of information. The only thing you need that's different from a normal aftermarket HU installation is a modified trim bezel to go around the larger display.