Hi Paul
Your almost certainly correct about the re-soiling issue.
So as to educate him on future spot removal procedures do the following:
Once cleaned, supply the owner free of charge
with just two spotters.
Solutions Eco Spotter & Other Solutions Woolsafe spotter who's name escapes me just now :

Together with these supply him with a
laminated A4 spot removal guide that has your name and number on it and inform him to follow the guide lines and only use these solutions to avoid any future damage.
Anything that can not deal with himself he should call you immediately.
Finally and most important tell him that as he is now your client that you will replenish his spot removers when empty, free of charge.All he has to do is ring.
He will be blown away by this amazing service you are providing and inform others in the trade locally.You will know that he only used approved products and this will assist you on future cleans.

Good Luck