Legally, you can send a registered letter to the customer stating that unless payment is made within xx amount of time (60 days where I live but I think 90 days elsewhere) the quilt becomes your property to sell to recoup the quilting fees. You'll be surprised how quickly they can find the cash!
If they still can't pay, you have to hope that the quilt will sell. Or donate it to whatever charity you like. Get a valuation from the charity for tax purposes.
I wouldn't be a bit concerned about losing a customer--she lost you as a quilter through her own actions.