I use 2 Revel SUB15's passives in my system, I'm sure these are what your friend has .....
"is explaining to me that when he pushes lightly on the diaghram of the cone it doesn't move at all."
I just went and pushed hard on both dust caps on the sub's woofers and "I COULD NOT MOVE" either one, repeat "COULD NOT MOVE" them ... I then pushed hard on the woofer at the very edge and I could hardly move it.
I then pushed on the dust caps of the 12 inch woofers on my NHT3.3 and they moved backwards without much force.
It appears the Revel's suspension is Ultra stiff ... run some Bass test tones through it when your friend is up and running again and see how it sounds.
Good Luck, Dave