Jarred,
If the spot was caused by a pH shift it typically needs to be corrected immediately, so you may be out of luck. If your cleaning chemical was the cause though, you should have pink all over not just in one spot.
If it is from a marker (yes, this actually happens far too much) there isn't much you can do about it and it really isn't your fault. Hard to look behind the fabric anywhere but inside a cushion, and I'd recommend that looking in the cushion be made a part of your inspection process. Often if there's a colored marker or adhesive used in the frame it will also be used in the cushions.
You may have uncovered a spill that stained the upholstery and is only visible now that the piece is clean. You have to develop a keen eye for things like this, and even if you don't see them prior to cleaning if you talk to the customer as soon as one shows up you'll be OK.