Surely the customer will be expecting you to be able to 'clean' the problem area and this is likely to be the reason she called you in in the first place.
If this is the expectation and you do not manage to fulfill her idea of the outcome with the products you have she may decide that you have not done the job properly even though the problem may not be a cleaning one!!!
Customers very rarely understand the difference between a cleaning job and a technical one (they think anything can be 'cleaned' better) so identification of a problem and being able to fully discuss with the customer the likely outcome needs to be done this side of cleaning and not after - just in case.
This type of stain removal (whatever it is) is not part of the general cleaning process (unless it will come straight off - which it hasn't) and should be dealt with as a separate matter and charged for accordingly, that way the customer is under no illusion that it is a simple matter to resolve it.
Most of these type of problems could have been prevented with the use of a leather protector (one that works efficiently) but unfortunately the customers are not told this by the retailer!!! :

Hey ho keeps us all in work!!
Hope this helps
Give us a ring if you need any further info.
Cheers
Judy