Some people have said it is the AVG certificate (though I find that a bit strange). It could also be her email settings are to display in text only - in which case the HTML code (which most emails are in) would be an attachment.
Is it possible to alter the AVG attachment at all - to text only say? - I can't look, as I don't use it.
You could prove a ppint by taking off the AVG cert - which is not necessary - and seeing what happens.