If I was paying someone to keep track of phone logs on a computer for me, I would certainly insist that they keep regular backups in case the computer fails.
Martha may have realized at the last minute that she was engaged in attempting to erase a message which might be interpreted as evidence of a crime, but possibly leaving backup copies which would prove that she had attempted to make the change. She may also have realized that a living witness to her modifying the phone log was every bit as potentially damaging as the original message.
Martha show every evidence of having arrogantly assumed that she didn't need expert legal advice. She could probably have copped a plea early on in exchange for implicating the broker. The circle in which she moves would not have tolerated such disloyalty. It is quite similar to the circle which protects the Klintons.