If colors are noticeably fading in a short time, the medium she’s using is not archival or suitable for long-term display. I doubt any kind of spray will do much to prevent fading of impermanent or non-lightfast colors. Inks and dyes are chemical and notoriously prone to fading much more than pigments used in paint. Most artist-quality paints have a lightfastness rating on the label. I suggest using different media that is more permanent and less likely to fade when exposed to normal room light.
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