The truck I've had experience with sounds very similar to your buddy's build...2010 6.7 Cummins. The truck trashed a stock 68rfe after 5k miles with nothing but a stock delete tune. It's currently running an S464, "150 hp" overs, dual fuelers, Hardway tuned, 48re swap/Suncoast internals and PCS controlled. It's a very healthy 750 ish rwhp. I just don't see how even the most extensive 68 build on the market would be reliable behind something at that power level. Those transmissions are just about worthless for anything more than stock power. Other 4th gen Cummins/68rfe trucks I'm familiar with seem to be getting 60-80k miles out of a stock 68rfe behind a completely stock 6.7 Cummins. Unimpressive.
To swap from a 68rfe to 48re with all the goodies seems to be hitting around the $10k mark. That's painful to swallow, coming from a $5k Allison that is good for whatever you want to do. I guess the trade off is they won't need a built bottom end for 800-900 rhwp. It's all the same money per horsepower in the end I guess, no matter the platform.