Hi Yoja,
"When you say assign an alias do you mean, create a new entry in the senders address book or change the actual email address that she is sending to?"
My suggestion assumes you have administrative rights to user accounts (Active Directory Users and Computers.) If you do, just go to the email tab and add another smtp address. Then ask your user to add this new address to the "From" line in emails to the problem destination.
Now that I understand this is an intermittent issue, I doubt that the alias will change the experience. But, try it. If messages are consistently delivered using the alias, then some sort of filtering mechanism is in place at the receiving end.
Next suggestion, try and remove the destination user from the process. Find out who handles IT for the destination firm and recruit their assistance (assuming this is not a lone-wolf user.) Too often people will state what they believe as if it were fact. I recently had a similar issue with mail being sent to a @pacbell.net address. Some delivered, some not. The distant user stated emphatically that no filtering was enabled on his account. The "@pacbell.net" user happen to visit my client's office for a meeting. I politely introduced myself and asked if he wouldn't mind my taking "a quick look" at his email config. I found the missing emails in the "Bulk Mail" folder. He just gave me the typical user "Oh!".
Other steps - find out the date and time of the next delivery failure and run Message Tracking in Exchange. Also, check your SMTP server logs (if external smtp server is deployed).
Let me know how you make out,
MAS