Hello Miguel Muelle,
For this concern, consider whether this happens with all email accounts, or only that one. Likewise, consider whether email messages show as read when viewing that account on other devices. Doing so may provide additional clues as to whether this is an issue with the Mac Mail app, the email account, or with a mail app on another device.
We'd like to share this information for reference:
Email messages you haven’t read yet have a blue dot or star (for messages from VIPs) next to them. When you select a message to read it, the dot or star disappears. If you want, you can manually mark messages as read or unread.
This information is located in Mark emails as read or unread in Mail on Mac.
If it's only happening with that one email account, try removing the account, then adding it back to refresh the account within the Mail app.
Remove email accounts in Mail on Mac
Add an email account to Mail on your Mac
Should the issue continue only in Mail on her Mac, test the email account when using a new user account. This step can help rule out something in her main admin account.
Set up users, guests, and groups on Mac
Kind regards.