@cindygordon granted me temporary FTP access to her site and I found a couple of things:
- The storefront /includes directory had been mistakenly uploaded to the /admin/includes along with the /admin directory changes. I'll note that this is easy to do (I've only done it a gazillion times myself), especially when there are both storefront and admin changes associated with a plugin.
- The store's Zen Cart version is 1.5.1, which is not compatible with current versions of EO.

I simply removed the EO-related files and restored the (thankfully only 2) core-file overwrites to their ZC 1.5.1 version so she's back-in-business and now looking to upgrade prior to the EO installation.
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