I got some good advice from the Chemmax people last night. After looking at the pictures, they determined that it is a Pakistani Chobi rug and most likely has been "tea dyed". The spill and customer cleaning attempts have likely impacted the overdye and even removing the yellow stain, if possible will still leave a discoloration. I think I will just return it to the customer and let them know that there are no affordable options. By the time someone tries to remove the stain and color it back, it will cost about the same as replacement it seems.What is the fiber type? Has it been cleaned yet?
Avoid an acid based tannin spotter. I've seen it turn the yellow to red.
I have an idea of what to suggest, but would rather have some history before I say.