My father is facing a difficulty that one of his employee do vacation once after two days. Even though he is a very honest and hard working employee, and also takes holidays for genuine reasons, it still affects my father's business.
There should be an employment contract that says how many days of paid holidays every employee has. If that isn't mentioned in the contract, then there are probably laws in your country that state the minimum.
Every employee has the right to take the holidays they are entitled to, and the reason why they take it is none of yours or your fathers business. Your father has to arrange his business to handle this. If they want more holidays, then they can ask their employer, and the employer either says "yes" or "no", and usually additional holidays would be unpaid.
The work location is helpful to know but in most developed countries paid holidays by law are between 2 and up to 4-6 weeks.
Anything more is usually unpaid or partially paid.
Furthermore holidays usually have to be agreed to by the employer and contractually or by law not guaranteed amounts won't need to be acquiesced by the employer.
Depending on the relationship and the employees mindfulness to the company it is always possible to plan holidays into natural business slumps or convenience for both parties.
If the employee is being unnescessarily difficult and frank conversations don't help there is always the door...