Calendar within mail.app?

All,

The company I work for migrated from Rackspace IMAP mail to Microsoft Exchange mail yesterday. One of the secretaries asked if she could see her Exchange calendar within mail.app. I've never seen the ability to so do any of you have any suggestions? I showed the secretary how her Exchange calendar is already embedded within the calendar app, but she wants to see it inside of mail. Are there any plug-ins that would do what she wants?

One thought I'm having is to switch everyone from the mail.app to Microsoft Outlook which I believe would do exactly what she wants, but this will cause a lot of fear and confusion for staff who have been using the mail.app for many years. Once again communications between the executives and IT department were not what I would have wanted before the boss pressed the GO button.

Note: the user base is mainly using MacOS 10.13.6, High Sierra, but a handful of users are using MacOS 10.14.x, Mojave.

Thanks,

Bryan