I have a user who tried to attach a file that's from one of their network drives. She is using internet explorer, and signed into Gmail. Windows 10. If she pulls up the "add attachments" window, they network drive isn't there anymore, even though it's accessible from her desktop. Any help?

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It's most likely Internet Explorers protection mode that is enabled. I believe the default is to enable it with Medium High priority with protection mode on. You can simply disable protection mode or go into the settings for the profile and enable that particular setting. Just not sure specifically which one it's pointing to in your case. Open Internet Explorer.
Click/tap on Tools menu, then click Internet Options.
Click the Security tab and then select the zone that you want to turn protected mode on or off.
Uncheck the Enable Protected Mode box, and click/tap on Apply.
THAT WORKS!