Generally speaking, the RIAS is more of a screening instrument than a comprehensive cognitive measure, with some evidence suggesting a tendency to inflate scores at the high end. I usually trust WISC results more than RIAS results, and, in fact, it is not recommended to use the RIAS for any kind of high-stakes decision-making. That being said, it is quite possible that a child might genuinely be underestimated on the WISC and more accurately measured on the RIAS.
One difference between the WISC-V and RIAS is that the the WISC includes Visual Spatial tasks in the GAI, while the RIAS is more similar to the Verbal Comprehension and Fluid Reasoning Indices. Was there any kind of diversity between her WISC-V index scores? The other big difference between the two tests is that the WISC-V has brand new norms, while the RIAS is 17 years old, which is good for at least 6 or 7 points right off the bat, and probably more, since there is some data suggesting that the Flynn effect (score inflation due to norm obsolescence) is more pronounced at the extreme right hand end of the bell curve. Unless, of course, you are talking about the RIAS-2, which was just released, in which case the norms are about equally new.
Another important factor is her age. Young children are much more likely to have large variances on formal testing, for many reasons, such as fatigue, inattention, not being in the mood, unfamiliarity with the testing process, feeling shy with an unfamiliar adult examiner, etc.
There is no way to know a priori which is the more accurate measure of her abilities, especially without any further detail (subtest performance, testing behavior, illness, emotional upset, anything that occurred that disrupted standardized administration, etc.). Though as a gross generalization, it would be reasonable to assume that it is more likely that one score is an underestimate than that the other is an overestimate. I am not quite making that judgement here, because of the screening nature of the RIAS, and its age.
Is there some reason it makes a difference? Such as access to services. Because if there are no high stakes decisions riding on her exact score, I would focus on the main takeaway, which is that she is a very bright young lady, who will need ongoing attention to maintaining academic challenge, based on her actual academic instructional levels in each subject area.
