Your friend is mistaken. The trucks are essentially the same except for wheels, tires, and suspension upgrades.
However, the additional weight carrying capacity, safety margin, and performance advantage of a dually make the 3500 DRW truck far superior for those who will carry a slide-in truck camper or tow a heavy gooseneck or fifth wheel trailer.
I can't remember all the inquiries I've read in over eight years of TDR membership from guys with 2500 or 3500 srw trucks wanting to haul a slide in or tow a heavy fifth wheel or all the guys who have reported that they traded their late model srw for a dually after realizing their mistake. I chuckle at the convoluted logic and strange arguments that attempt to defy the fundamental laws of physics expressed by some who buy a srw truck then decided later to purchase a heavy camper or trailer and discover to their dismay and deep regret that their truck can't safely carry the load.
Some will show up now and angrily defend their srw truck and that is their right. The fact remains, a dually can legally and safely carry/tow far more weight than a srw truck.
Each to his own. Present your arguments my friends.