thin/fine thread, small needle, loosen top tension a little bit- tell her, polyester sheets are *ok* for tied quilts- but not so great for long-arm quilting- you have to slow way down- check constantly for skipping stitches-- most of the long arm quilters I now just say no- sorry- polyester sheets really are not well suited for this purpose.
if you really want to do it for her it's going to take you a while...slow & steady- going back over areas often ,,, if you are long-arm quilting for customers you should have policies written out- stating what you will accept-
what you are using for batting also plays a part. you 'might' be able to stabilize the polyester sheet with spray basting it to the batting, or adding a thin lightweight fusible interfacing- but that isn't going to make a lot of difference with the skips- just with it sliding around.