Please do not follow the suggestion to do the work for your GD. If you do, you are teaching her that it is quite all right to cheat and afterwards to lie to her parents! I'm sure all the posters who suggested this course did so out of pity for the child, but jumping from the fat into the fire is not going to help.
If she were my grandchild, I would if possible say to her parents that this stupid maths system is counter-productive, as it is making the child simply hate the subject, and as she will have to do it for the next ten or so years of her life that is decidedly not a good thing.
You say your DIL is controlling, so maybe this is not an option, but surely if you are tactful and tell your DS and DIL that you really have tried to follow their instructions, but that you feel it did more harm than good, you could get them to discuss it. It does of course mean you have to avoid the slightest hint that you are criticising DIL.