There are cases where some employees during the probation period had to take frequent leaves, however once they crossed their bad phase proved to be real assets to the organisation. You need to decide the matter on its merits. However, if leave was sanctioned in the past instances, then no action can be taken on it. In case you are satisfied on the genuineness of the medical grounds, you can consider the request and allow LWP/EOL and regularise the absence. However if it is felt that the employee on probation is unfit for the organisation, then take action to discharge the probationer on this ground.