Slate does contain iron which can oxidize, however with that said I would first have the water checked for iron. You can buy a test kit at home depot or lowes> If the iron is coming from the water, than you will need to tell your customer to install a filtration system. If the iron is coming from within the stone, the only thing you can do is clean it and when it returns clean it again. a solution of oxalic acid should do the trick to at least remove the surface iron