Seeing as you're going tomorrow and don't have the chance to possibly order checms from the online sales people, go to your local chemist and get a small quantity of H202. Downunder we use 6% but not sure what that equates to in you %vol calculation.
Dry vac the rug really good, both sides each twice. Apply the H202 to a test area and check for colour loss/change etc. If ok, apply liberally to just the immediate area affected. Do this while the rug is still dry as it will gain the most benefit.
Sometimes a light misting of ammonia over the top will activate and accelerate the process. Leave for 5-20 minutes and maybe even add a little more to it.
Rinse thoroughly with a mild acid rinse solution, apply a good wool detergent (ok so you don't have any, go into your local grocery store and in your laundry department, buy a product that is designed for wool or ladies delicates) apply it and work it through gently and extract again on the acid side.
Be careful with H202 that you wear gloves etc and don't drip in anywhere it shouldn't be.
Once that is done, clean the entire rug 9with the sme product if possible) and groom and speed dry. As mentioned, leave on the acid side and you should be ok.
Sometimes you don't need the expensive orders to get you out of trouble, especially if it's not something that you will use a lot of. Those two products will set you back $10-15 total, just add a slight surcharge to the job.
Don't undersell your service though, even if she is a good customer. By all means charge a reasonable rate for the cleaning but make sure you charge seperately and appropriately for the stain treatment.
Obviously there is the qualification required before committing to the job, like alkaline salts will cause permanent damage to wool if left too long in place. You may reduce the look but may not get that last yellowing... safely.
Something else you may want to try if the above doesn't work would be a little of a sodium metabisulfite solution. I assume you will have something already in your spotting kit with that in it... offensive smelling so make sure you have adequete ventilation. I know Prochems Coffee and Tanin remover is that base. Just make sure you have THOROUGHLY rinsed the H202 out PRIOR to applying the sodium metabisulfite solution.
Take before and afters and let us know how you get on.
John