How does she do if she has a diagram she can literally see, labelled -10, -9... 9, 10? Does having it as a thing she can see in front of her make any difference? How is she on graphs with positive and negative values on the graph on both axes? Plotting it out and writing it down might become METHOD, but then I still multiply large numbers by drawing a table, which was the only method that ever stuck for me on that one.

Otherwise, there's repetition and memorisation maybe, to get a table of values as a mental reference? That's how I do C/F conversions... badly, but I can't ever handle remember the formula to multiply, so I have to make a rough chart and guess from memorised values.