Why would it be hard to put it on her server? I don’t know how these things work?
I was joking, (only maybe less than I thought). To put an application on a server like that requires administrative access. Somebody with that had to want it there. A hacker wouldn't bother - if he or she had administrative access there are FAR more interesting things to do than install a disk wiper.
This gets a bit technical, so forgive me. It was a Microsoft Exchange server, by all accounts. Deletion of individual emails doesn't actually involve file deletion - they're kept in a large database that is the mail store. If they're exported to a file for some reason, that's different. Mail administrators often do that to give the user a package of emails in a file, which remains on the server after it's copied to media that is given to the user. They'll do that to satisfy a subpoena, for example.
You'd use a wiper to delete those export files, if you needed to. Or, you can use this wiping software on the entire mail store, which is what I suspect happened. That's very naughty indeed. At that point all you have left is whatever emails you've already extracted and stored off for, say, a legal discovery process. If you didn't do that to all of them and you've wiped the mail store, you've covered your tracks very nicely. What the software they're describing actually does is delete files and write new bits over the old area, delete them, rewrite, delete, etc. It makes the original "fingerprint" impossible to recover...well, not impossible. Just very difficult and expensive, and you'd need somebody really good to do it. Which the FBI certainly has.
I hope that's not complete gibberish. There are other ways to recover email that entail access to outside routers and logs. Once it's sent, there are bits of it all over the Internet. Again, you'd have to have deep access and be very, very good to recover email that way. The NSA is very, very, very good.