To my knowledge, some companies like yahoo and hotmail have their own virus scanner it runs before receiving and sending messages with attachments so maybe it is the programs itself?

Has she tried sending any other type of file or only the spreadsheet? Can she try sending an email with a small image or a different file type just to narrow down teh problem?

Is she using an email PROGRAM or the clients through the website??

And if it is through the website, I am assuming she is using internet explorer.. make sure she is using the most current version of her browser.

All else fails, try in a different browser?

Or.. another idea... does the problem only happen on her computer? Can she log in to her email accounts on someone else's computer and send attachments without a problem?