It's always something! I think I would tell the woman about your concerns and that you are willing to try to complete the quilt (but only if you are). I would definitely prewash all 100% cotton fabrics and shrink them as much as possible before cutting anything. I would also use interfacing on the polyester fabric to stiffen it up a little. I probably would still use 100% cotton thread for piecing & quilting. I have mixed different fabrics in scrappy quilts and haven't had any problems, but I prewash everything and I think that is why mixing fabrics works fine for me. Good Luck!