The first step is to positively identify the type of stone. There are a number of products that could be helpful on some types of stone but would be harmful to other stone surfaces because of the products acidity.
Also determine the gloss level your client wants. Cleaning is often not going to correct this problem. The cleaning needs to be followed by honing and/or polishing.
You can send me some photos if you are having trouble identifying the stone. I also have a training guide for Stone, Tile and Grout that I can email you. send me an email [email protected] and I will attach it to my reply. Put stone reference guide in the subject line, please.