At this time, we haven't received any reports from other users about customers being unable to open invoices from their email. May I ask if your customer encounter error messages when trying to open the invoices they received? I appreciate any details so I can present accurate information.
In the meantime, I suggest advising your customer to access the invoice received via a private or incognito window to check if this is a browser-related issue.
You can use either of these keyboard shortcuts:
For Google Chrome browser: Ctrl + Shift + N
For Mozilla Firefox browser: Ctrl + Shift + P
For Safari browser: Command + Option + P
If it works, then the main browser may have too much historical data from previously visited websites that cause this kind of behavior. That said, your customer needs to go back to the regular browser and clear the cache to delete those temporarily stored files and browsing history. They can also use other supported, up-to-date browsers as an alternative.