Hey there Nick. I've found that sometimes the accessibility tags are kind of wonky (those are there for use with a screen-reader). If it's an acceptable solution for you and your colleague, I've found that deleting these tags in the file eliminates that error 100%, at least in my case.
To pin "Tags" to your navigation pane, go to:
View>Show/Hide>Navigation Panes, then click on "Accessibility Tags"
There will now be a tag icon in your left navigation pane (if it wasn't already there). Now, whenever you or your friend are having that error, simply click the tag and then right click the upper most tag in the tree (with the PDF icon) and click 'Delete tag'.
I don't know if this is a fix-all, but it's worked perfectly for me! I have that error all the time and it drives me nuts so I know the feeling.
Hope this helps!!