Hi Lydia!
I’ve never scanned artwork before, but I have scanned images (and edited them) and GIMP has worked well for me in the past. My siblings all use Mac and there is a photo editing program on there (not sure what it’s called) that seems to work well. Scanning does make the colors less vibrant, but I think some saturation could help with that. Also, your cousin may want to read the printer’s manual (if she hasn’t already), since they might have suggestions or different settings that would work for higher quality scanning.
Hope this helps!
-Erin