When someone edits an email from inside a campaign set up it does not actually edit the template where that email came from. The template remains intact, as it should.

The issue is that the labelling in the email builder is misleading and causes confusion, as the client thinks they are editing the original template.

My recommendation is to change the labelling as shown in the image from "Edit Template" to "Edit Email". If you are in the template section, the "Edit Template" labelling would remain.