You have not mentioned the status of your firm. Are you a factory, a shop, or something else? Normally, Privilege Leave (PL) (in some establishments, it's Earned Leave (EL)) can be utilized for one day or more (not in fractions like half a day). If she has only 2 days in her credit, what prevents her from availing one day or both days together? There is no violation. Generally, there is no link between Casual Leave (CL) and PL/EL; they are not interchangeable, except in special circumstances. It is not a common rule that "unutilized CL can be converted into PL/EL."
For example, if an employee was not granted any CL at all or there are more days in the credit of CL at the end of the year due to the inability to avail/utilize them because of official work pressure (otherwise, unutilized CL automatically elapses, and hence the employee totally foregoes it for no fault of their own). In such cases, special orders are passed to carry forward unutilized CL and merge it with PL/EL either 1:1 or 2:1, etc. This type of arrangement should be part of the firm's leave policy.
If discretion is given to appropriate officers in this regard, administration becomes easier to regulate granting leave vis-a-vis workload. In firms that follow the government's pattern of leave eligibility, CL is not merged with either PL/EL or Sick Leave. However, if the leave policy delegates powers to designated officers, this can also be relaxed to manage critical situations like hospitalization, etc.
Above all, you have to consult your own leave policy first before consulting the policies of others.