who is at fault? the mason contractor or the material supplier?

by cathy
(illinois)

Provided were two sided, two header bricks. There are rust band marks on one side. The mason layed the "rusty side" out, and is looking to the supplier for a remedy.
Supplier recommends an additional cleaning with a special rust inhibitor solution. Mason contractor states that the rust was not visible during construction, and that they have "already cleaned the brick using conventional methods" and that it is the liability of the supplier of the material to remedy the situation.

Who is correct?