Hi Mukherjee,
Casual leaves are defined types in various organizations. Casual leave is not a recognized form of leave and is not subject to any rules made by the Government of India. Casual Leaves are essentially intended for short periods. In many companies CL will not be granted for more than 5 days at any one time, except under special circumstances.
It also depends on the leave policy of the organization, wherein they allow CL to be prefixed or suffixed with public holidays and weekly offs. When the employee herself requests for a leave without pay, then I don't think you should have a problem in that. Again I don't have much clarity in this case, in regards to the leave policy of your organization. Hence my answers have limitations.