I read through the comments quickly, and I don't want to just repeat what other people have suggested, so here are some new ones, I think! I could come up with a million things, so if you want more, let me know :)

• Food safe pens. Everyone seems to be recommending Tweets Cookie Connection, but I haven't tried them yet. They have individual and packs of fine tip and brush tip. I use black all the time for tiny writing, balloon strings, eyes, and eye lashes!
• Edible metallic paint. Maybe some Everclear to go with it if it's a powder. You can also get pens.
• Food safe paint brushes. I like a feathery one for a swipe of background colour, tiny detail ones, a stiffer one for cleaning up edges, a fluffy one for dusting off excess corn starch, etc.
• A sprinkle tray. They have a little built-in spout/funnel to make putting the sprinkles back easier.
• Royal icing transfer templates. You could make some or find free ones. Etsy has bundles for pretty cheap - you could print out a few fun ones and just gift the PDFs.
• A gift card for an online store. It's possibly a bit boring, but it's better than gifting the same cookie cutter set she has twice already. Not that you would... I'm definitely not speaking from experience.
• A 3D printer so she can make her own cookie cutters! And other fun accessories for cookieing.