Why?Do you doubt? Just apply this theory, when he is on leave without pay for the entire month, he will not be paid salary or the salary will be ZERO. When contribution is payable at the rate of 12% of the salary paid, the contribution is equal to zero or 0 X 12%= 0.
An employee on ESI leave will not be paid salary from the company and hence the salary will be zero. It is immaterial whether he gets sickness or accident benefit from the ESI Corporation. For contribution the payment should be made by the employer.