If she has vacation days/personal days allowed per her contract she is allowed to take them for any reason or no reason. Typically people entitled to those.

Of course it’s preferable if she schedules them ahead of time, like a month in advance, not a week. That could be frustrating. So I understand that short notice is irritating.

But other than that she likely has vacation days she can use here and there, not necessarily going away on vacation. That’s likely what she uses those days for: birthdays or hobbies etc It’s not against the law. When someone dies, those are likely separately allowed days for bereavement. When she is sick, those would be sick days. Often separate from vacation days

Her having Saturday and Sunday off doesn’t mean she has no vacation/personal days in her contract. She very likely has those days like everyone else. In many work places if you don’t use these days, you lose them. So it’s often encouraged to take those days for whatever private reasons.