A stock (as delivered from the factory) C36 runs in stock

AMG cars that are stock (the way they came from the factory) run in stock classes -- IF however you add wider wheels and or larger diameter wheels and a set of competition tires you will now be in the Prepared class -If you alter the exhaust or do anything on the list of modifications in the rules you will move up to modified or race depending on what you do and points assessed . Maybe I am confused about your friends car .
Is his car stock from the factory ? If his is not a true AMG production car then he would have to add all the modifications that were done to the car after it left the factory. If his car is an authentic AMG product then he would run in the stock class .
In high speed events the A class Mercedes are
C36 AMG, c46 AMG , C43 AMG, E55 , AMG all models

B-class Mercedes are 600SEL, 600SL,S500,500E all
and the E430 all (all meaning all variations of that model and years
C-Class Mercedes - 400E, E420, 500SEC, S430, C230 Supercharged, CLK320, all
Mercedes SLK all