Oh, I can imagine how bad you must feel. If I were your customer, I'd understand and try to make you feel better about it. Before you do any of our suggestions, I think you should let her know and tell her that you are trying to fix it.
I would try a powdered oxygen bleach mixed with hydrogen peroxide. I used this with terrible old wine stains on white fabric Purificators (Church napkins used for Communion). It worked after I prayed a lot.