Hello Kristen - Does the user have more than 1 Work or School account associated with their Windows computer?
Search for the 'Manage your account' system settings
Click on the Access work or school
I have seen if they have more than 1 account listed here then they need to disconnect the one that isn't being used for Business Central. They probably have more that one account due to adding an additional account through the desktop Outlook program.
If this does not work here is a blog that Chris published on troubleshooting steps with Edit in Excel.
blogs.msdn.microsoft.com/.../