Definitely agree, this is not a Machform problem.
The user needs to delete the auto-fill entries in his browser settings. Given he knows what "auto-fill" is, and blames it for the errors, then he could easily figure out how to delete it.
You can't control how his browser saves the form values (or not) since that's an end user setting in the browser.
You could refer him to something like https://www.pcworld.com/article/3145489/how-to-clear-unwanted-autofill-entries-in-google-chrome.html or https://www.thebalanceeveryday.com/how-to-delete-unwanted-auto-fill-entries-in-your-browser-896875
However he could also try filling in your form in a different browser that doesn't have the auto-fill (wrong email). The auto-fill data is browser specific - so if the auto fill thing is occurring with FIrefox, advise him to use Google Chrome or whatever.