You won't be believed.
The thing is, the test time is done under supervision (at least it usually is). It's always a good idea to get a copy of the test, and check it at home (I wasn't autistic, but I usually got a copy of the test after the exams and used it to find where I was wrong). But it's what was completed under supervision that gets the marks.
So, give her the benefit of the doubt -- if she is cheating as you say, it will eventually come to light, and she won't benefit from it.