Not Really Sick
If you have to lie about being sick in order to get a day off on your 21st birthday, then we think you need to change your job. Thankfully, this person thought that too and acted on it promptly. A workplace is not a jail but some employers make it one.
You can be as professional as you want without denying little happiness to your employees. In fact, the best workplaces are those where the employees are valued enough to allow them to celebrate their special days. There is no reason this person shouldn’t have quit.