From past experiences the holiday is calculated as pro rata over the holiday year to the date of leaving, and any overtaken holiday is deducted from any final salary due.
It may be worthwhile checking employment contract. If there is not enough salary in the final month to deduct from an overpayment calculation may be done and she may be requested to repay the net amount owed.
I cannot guarantee anything as each company operates differently - this is just what I have seen happen on most occasions.