I'm researching for my buddy who's on his way home with his truck that threw the 0735 code while cruising down the interstate. He has a 6.6L Duramax with a stock Allison tranny, he has a PPE tuner. He said he had it on level 4 for the past couple weeks and has been driving it pretty easily. Today while going down the interstate it dropped into 4'th gear out of the blue and gave him the dreaded "0735" code. I think the truck has around 70k miles, he bought it with 60k and put the PPE in himself. He doesn't do any outrageous towing with it, just our ATV hauler which we take turns (I have a Ford F-150 with the smaller V8). My truck handles it fine so I know it's not what's causing his issues.

What could be the issue with his tranny? I know the code means that the 5'th gear ratio is incorrect (ive been searching), which could be caused by a slip in the clutches (I believe) or the torque converter (right?). Should he start out with a fluid/filter change and go from there? Or is he doomed to a rebuild?

Thanks in advance and don't hate me for my Blue-Oval, I'm not truck-racist and I hope you all arent either.

Sounds like he limped it. "Slipped the clutches"
Have him set the PPE down to one of the lower tunes "Tow tune" or back to stock. He should be OK for a while but should start a tranny fund.