I just went through the same thing with my sled. Its an 07 M8 as well. I checked everything, sensors, put in new PV cables, checked the Servo, swapped secondary clutches, checked primery spring, swapped out weights on the primary, you name i checked it. got fed up and took it to the shop. they narrowed it down to the Stator after they rechecked everything i had checked and a few things beyond that (oil pump, etc).
It would not go over 7000 RPMs and had no top end. ran fine throughout the rest of the RPM curve.
Gonna get replaced on wednesday.
I would bet thats what the problem with your buddies sled is.
I had a thread on this not to long ago titled, 'Did i blow my top end or PV cables' check that one out. i went through and checked everything everyone suggested as well as the troubleshooting points in the service manual. but no one and nothing directed me to that stator
I have about 2500 miles on mine so if your buddy is in the ball park on that mileage it could another indicator.
I dont know how to test the stator or replace so i am no help there or how/why they go bad. I sure would like to know tho.
good luck