I, too, would have gotten involved and spoken up. After making draperies and other window coverings for a long long time professionally, I would have opened my big mouth. I have in the past and believe I saved a customer some heartache and disappointment. AND, I always receive the input from others concerning my shortcomings.
I think, too, that you should speak to management about the training of their cutting staff. If there is a question that she/he cannot answer, they should seek help from management or other experienced cutters and this should be pointed out to management. These stores are to keep us happy and obviously, they fell short in this case.