Not sure, but I am sure if you are putting x amount of sub/s in there off x amount of power it is going to rattle some. I would just tell him to check into some deadner, which there is many brands out and deaden the trunk and take it from there.
Only car I have had that didnt rattle the trunk much was my cutlass with over 4000rms off 4 12's, lol! How you ask, well box was built in trunk and plexiglassed and the rear deck was molded for my entire port.
Now ask me how bad my back window vibrated and broke and how the top of the roof would wave at me, lol!