If I remember correctly Buff All acts as a buffer (stabilizes PH), however I doubt it is a reducer as that will neutralize the oxidizer. I would suspect it is an acid (not sure.)
Why use a reducer on that stain to begin with, before you know (and tried) that the color damage is actually not removable?
If you do not have a neutral cleaner, get one :shock:
Unless you'd really like a linen sofa in your home.
Chocolate is a complex stain. It has grease, protein from milk, cocoa butter, sugar, potentially some dyes etc.
How big is the stain?
Can you safely scrape some out, if yes, do. Do not push it further in. Vacuum to remove as much as possible.
As said above, stay on the cold side. A VDS may help with the grease but like any liquid it may help dye penetrate the fabric.
Enzymes may help with the protein. Still a liquid and on some fabrics, tends to lighten things. A neutral cleaner may be the first cleaning choice. If it does not work and if the fabric tolerates it. sometime an amoniated pre-spray may work. After that there are the dye stripping options.
Test Test Test.
Have client sign a release of liability before proceeding.