We took my mother to an 'over 70s check up' - obviously, that gets round the "there's nothing wrong with me hurdle" as it's a general MOT for those of that age, and most elderly people are used to this idea as the GP tends to call them in from time to time for that type of thing. As others have said, you can send a brief note outlining the difficulties ahead of time. You could offer to accompany her and then go shopping/for tea afterwards if you think it would help.
Bear in mind that this will be the first of several appointments. In order to get a diagnosis there will be at least one appointment at the memory clinic, possibly more (and there is no disguising what the memory clinic is!) plus an MRI scan. So you will need use persuasion to accomplish that, and she will almost certainly need accompanying to get her to attend.