You're probably right that the issue is on your friend's side. Either her ISP or her email program (or both) seem to filter incoming email. Some such filters may be bypassed by her adding your email address to her address book or equivalent.
Btw, is the filtered content (exclusively) text or image? Image filters may offer the option to load the filtered images, usually via a button or clickable text ['load images'] at the top of the email. It sounds like this is the case here.
