Sounds like overboost (resulting in fuel cut) under high load in that rpm range, which makes sense if he is running a stage 1 tune with a downpipe. Now that the temps are dropping, it's much easier for it to occur.
I can help point you in the right direction remotely if he can post up logs of the issue occurring. However, I still recommend you let someone with mechanical/tuning ability actually lay hands on it.
Log the following values:
Engine Speed
Engine Load
Feedback Knock Correction
Fine Knock Correction
IAM
A/F Learning #1 (%)
Manifold Relative Pressure (corrected)
Wastegate Duty Cycle
Target Boost
Injector Duty Cycle
Mass Air Flow (g/s)
IAT
Wideband o2 sensor if he has one (and he should)
My advice to him is to stop driving the car, or to stay out of the boost until this is sorted out. I realize on one hand I'm telling you to post up some logs of it happening, and on the other telling you to just take it to a tech. I'm trying to offer you options, in the event that he doesn't live around a competent tech. If you tell us where he is located, we might be able to recommend someone he can go to.