I don't have your exact answer, but just a direction focus to employ. Since you've not gotten an answer in over a day, I decided to post this to see if it might help.
Most likely the problem is that she appears to the Postfix server as an outsider sending email to an outsider. That's relaying and it is rejected because spammers have used that to magnify their efforts. Have her try sending test email to a user on that server to confirm that it works that way.
There are a few ways to fix that. If she is on a local LAN, add the LAN address to list of local networks in the main.cf file on "mynetworks =". If she is on an outside network, you need to set up secure submission and user logins. There's lots of different ways to do these things depending on your circumstances and the details of how you need to deny and allow access to others.