Your Career Here
There is not much confusion between a career and a job, or is there? It looks like this employer thought both terms meant the same thing, which obviously isn’t true. You cannot expect your workers to leave everything behind in their free time and come to your rescue.
It is a manager’s responsibility to handle every employee’s shifts in such a way that nobody has to give up their day off for some other employee. Besides, the employee had a complete right to deny any extra work shift if they had another job to manage as well.