Do other people see that error or just her? If only she sees that error, I would suggest validating her permissions for the datasource. If multiple people have that same error, it looks like the Tableau Server instance cannot access the datasource. Were this not Tableau Online, I would suggest checking the Run As User account permissions, but that is not a valid solution with Tableau Online. Did you open a case about this issue?
Thanks for the reply! She is not the only user, I checked other interactor accounts and they couldn't access that specific one either. I checked the datasource permissions and they were the same as all of the other ones that she is able to access. My alternative at that point was to delete the data source and workbooks from Tableau Online entirely and republish both from Desktop. This didn't work either (and she's still able to access all other workbooks in that folder).
I have opened a case about this and am awaiting reply.