Most isp's let you logon to their mail server from a web browser. Try & see if her email is there.
How are you receiving the email? OSX Mail? From a web browser?
Have you looked in the trash?
Try locating her email with spotlight. Might tell you where it was filtered.
Perhaps her address got on some filter list.
mail > preferences > junk mail
mail > preferences > filter.
Turn off junk mail filtering as a test.
edit all filters so you can see what they are.