The question I would ask is did the customer try anything on it first? I don’t understand chemically how a stain gets “set in”, but I constantly come across stains that are set in by store brands (Resolve, Spot Shot, etc.). I can get out the splatter that these products didn’t touch, but the main body that was saturated by it won’t budge.
Unfortunately I am with Loren and haven’t had much luck with yellow spots (see the Ranch dressing spot I lost to a few weeks ago). If you didn’t try them, my only other suggestions would be blood remover for the proteins in the chicken/broth, or TGP/dry solvent if grease is the problem. Good luck.