How do scammers insert the change into a return address for a scammed email?

Just recently I received an email purported to be from a friend who actually had recently gone on a trip, asking or money to help her get home. The email return address differed from hers only by the insertion of a “1” atter the part before the ”@” symbol.

How do scammers do this [not planning to try it myself]—insert the change in the return address? See code below:

Message-ID: <1345570696.52799.YahooMailClassic@web126001.mail.ne1.yahoo.com>
Date: Wed, 19 Sep 2012 03:31:36 -0700 (PDT)
From: Ann Nonymus <heraddress@yahoo.com>
Reply-To: heraddress1@yahoo.com [how do they insert this?]
Subject: Horrible Trip…....Ann Nonymus
To: undisclosed recipients: ;

It is an old scam. I wrote about it in my blog 2 years ago .

Any ideas?