I haven't had that happen to me and I have quilted many T-shirt quilts for others.
How far along are you on the quilt. I would call the customer and let her know what is happening and ask if you should continue. She could probably fix it by using a pressing cloth on the back side of the top.
What batting are you using? Some battings help the top cling to it a little.
I understand how you must be feeling because your quiltling is your reputation...as good of a quilter you are it is difficult to correct some piecing errors that are beyond your control.