Welcome and thanks for posting in the Community, @usermorgan.
To check if this is a browser-related issue (cache and cookies), have her sign in to QuickBooks Online (QBO) using a private browser (incognito).
If it's successful, she can return to her default browser and perform a clear cache to refresh the system. However, if the issue persists, have her use other supported browsers.
To learn more about managing users in QBO, consider checking out these articles:
Please leave a comment below if you have any other issues or concerns. I'll be more than willing to help. Have a good one!