I towed 12k (37.5' 5er) and 14k (39' 5er) with my 2014/6.4L Hemi/2500/4.10 truck. It was rated at around 15.6k lbs. It did great with both. The 12k was no problem at all up and down any mountain pass I chose. The 14k I only towed as a test on the flats but didn't notice a performance difference.
I can understand his frustration with the problematic, expensive to fix diesel he currently has. So long as he's willing to let the 6.4L Hemi use the higher RPM it will need, it'll be fine.
That said, I'd expect all 3 truck manufacturers to come out with new gas engine offerings for their 2500/3500 trucks in the next 2-3 years, so he may want to wait.
