Outlook Web Access (OWA) does not have a security line. The only way that I have found is to configure a second user to have rights to this account, give this new user enough rights to open the account and share the target user's calendar using Microsoft Outlook.
To assign permissions on individual folders like the Calendar, you would need to log in as the user. Alternatively, you could use ExMerge to export the user's calendar folder/mailbox and let the other user have the .PST file (or copy of his/her mailbox).
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