Well I talked to my friend but he doesn't remember what the stuff was other than it was a mild acid solution.
A little surfing on the web netted this:
http://www.prowax.com/1prod_bulls/C-30pb03_12_04.pdf
C-30 Water Spot remover:
Easy to use liquid removes water spots, etching & pitting from glass, chrome and stainless steel surfaces with very little effort. Mild acid formula breaks the bond between mineral deposits and the substrate. Leaves the surface free of deposits while restoring the optical clarity of glass and other surfaces.
From reading the instructions and description it sounds pretty close to what he described.