I'm guessing you already have one. If not, get one. Then, chose:
Option 1: Tell your buddy he's right.
Option 2: Tell your buddy "I understand your point of view, and that may be the right answer for you, but I've reached a different conclusion. "
Option 3: Tell him that your poor little inferior Cummins needs the exhaust brake because it doesn't have the "warm up" feature built into the Fords at the factory. Then every year or two, share a few experiences and ask him how his brakes are holding up.
He can "get by" without one - lots of folks do. Sure, properly used, under some circumstances, it might save his life. But if we want to talk about that, then what about improper use? What if he has a wreck because he used it when he shouldn't have? Will that be your fault because you talked him into it?
All I can guarantee you is that even if he "gets by" without one, you'll continue to be glad you DO have one!