To check for rules on webmail log onto the BTMail account using a browser, not an email client/app, and then click on her username at the right hand side. Select "Settings" then go to the left hand side and select "Mail". Scroll down and click on "Rules". It should be obvious to see if any rules have been set up.
If there are any set up that then begs the question "who set them up?".
If it only a folder named "sent" then I would do as I suggested and move them all to her sent folder and delete the old sent folder. It serves no purpose other than as a storage folder.
