No Longer Needed
Working a double shift has to be one of the worst things in this world, but there’s a way to make it even worse! Ask the employee to come in for the second shift after they just got back home from their first one. Understandably, the OP refused to take the second shift. They had stuff to do, anyway.
However, that was the last shift they ever worked at their old workplace. How does it make sense to fire someone for being unable to cover someone else’s shift? Well, OP, you’re much better off without having to work for such a senseless boss.