I just do not believe this story. Nobody in the right mind will fire key software engineer 48 hours after system goes live. This is where all the bugs/problems show up and you need all hands on deck to fix them. Six months after launch when all problems are solved - absolutely, you can fire engineer. 48 hours after launch? absolutely not.
Perhaps there was a backstory of sorts we're not aware of. But there is absolutely nothing surprising about companies firing people right after handing over huge and/or critical projects. They do it all the time, for both good reasons and bad. Managers in finance are known to be especially "bold" in making decisions like these.