Got any codes? He might be revving the piss out of this thing and causing valve float.
Also worthy of note, the exhaust valves can creep open with excessive EBP which may be a by product of the tuning and not running a waste gate and high amounts or boost.
The 6.0 will help valves close every time. Lol.
I would expect it to bust up the rocker arm as well, but wherever the weakest link is would be the result I suppose.
Are you changing the whole rocker assembly or just the rocker arm and clip? I had one that the mount that bolts to the head was just wrecked underneath from the rocker arm bouncing off of it.