How tall is she? You want to make the throw big enough to let her cover herself in it and snuggle comfortably. I think 70 x 70 might be a good size. Or you can make it longer than wider instead of a square. Some people make lap throws smaller, but I like them to be generous in size so you can really snuggle into them. You may want to consider backing the quilt with flannel or fleece to make it extra snuggly. You are doing a wonderful thing for you co-worker.