Uuuhh???
Correct me if I'm wrong, but applying wind to TAS gives you ground speed, not a different IAS. I can't see that your Avcom man's reasoning is correct - but then again, I might be wrong. Rainboe's reasoning makes more sense.
Using your Aristo, you can see that at that height, and temp with 100 IAS, your TAS should be 113kts (as shown by your Avcom man). However, your real TAS is 120kts (109kts out and 130kts back with 11kts wind component). Therefor, using your Aristo, 120Kts TAS equates to a CAS of 105Kts. This gives you a 5kt under read. That would be my reasoning, but it seems as though my answer is incorrect.
CHEERS!!!
I