Hi ,
Remove all the existing permissions on the target folder and use the below mentioned command to provide the read permissions.
Command to remove the existing permissions on the target folder :
remove-MailboxFolderPermission -Identity "nithyanandham.s@abc.com:\test" -User "kumar.n@abc.com"
Command to provide the Read permissions on the target folder :
Add-MailboxFolderPermission -Identity "nithyanandham.s@abc.com:\test" -User "kumar.n@abc.com" -AccessRights "reviewer"
Reference Link : http://www.slipstick.com/exchange/view-shared-subfolders-in-a-exchange-mailbox/
Before doing all the above mentioned steps please read the entire article which is mentioned above and it contains detailed explanation.