If she is fearful of testing, how afraid would she be if someone actually attacked her full of hate and loathing? To me this is one of the most frightening things to experience, and testing in a very severe atmosphere is the easiest way to apply stress.
Aikido isn't just about doing technques; it is about self-defence. I would worry that such a personality may freeze up given a real situation. Therefore this problem is something FUNDAMENTAL which must be overcome. Maybe see why she is afraid of testing. Maybe she has to overcome a fear of failure, an embarrasment of people looking at her. However, I would certainly try to get to the bottom of this, and not just an easy cop out of lessening the pressure during a grading or not requiring a grading.
Sounds ruthless, but I think everyone will be better off at the end of it. (What you could do, is do a grading within the normal class - if she fluffs it, fail her. Then do it again a few weeks later (maybe just for her) etc until grading becomes routine for her).
Eventually she will get bored of worrying.