I would definitely check with the customer before I washed it. Did you prewash the material before making the quilt? Even if you did I would also use retayne if I were going to wash it or have the customer wash it using retayne that you might furnish. After using the retayne, then you will need to wash it like you would any other one that just needed washing. HOWEVER, my suggestion to you and to the customer, whoever washes it after using Retayne, I would also put at least 2 color catcher sheets in the wash even if the fabrics were pre-washed before making the quilt ASK ME HOW I KNOW!. But I would definitley check with the customer first and get her wishes. You might explain to her that when quilting you do handle a quilt constantly, and even with clean hands and workplaces, body oil can soil any thing, and dust from the air, no matter how clean your home is can accumulate on any area. Don't dryclean, that might set the dirty corners in permanently.
I would try as someone earlier posted to wash with a damp cloth just one corner and and press well to see how that turns out, before making an effort to wash the entire quilt. You may just have to wash each corner. If washing one corner doesn't work, then definitely check with the customer before washingthe entire quilt!!
Can you show us a pic?