Yes - sign up for a hosted Exchange provider. No - there aren't any other solutions.
No offense to the other posters, OP, you'll need either access to her Mac or the local server her Mac synchs with and Outlook (2011 or 2016) or set up an Exchange account. Outlook - which I use daily - either uses Exchange Web Services (EWS) for calendaring or a local calendar, and neither version of Outlook offers an alternative. Furthermore, there's no direct means to sync Outlook to a cloud service that is not EWS-based. MS has designed the Mac versions of Outlook to work this way - period.
You indicated that IMAP is used for email. Therefore, the only option available for calendaring is either access to her Mac or a local server, and I'm guessing that it's the former option as it's far cheaper (per month) to set up a hosted Exchange account than run a server just for calendaring.
In Outlook, the only other option available is to physically drag-and-drop calendaring events to a location (folder/directory) - but this action still requires either her to drag-and-drop, copy to the EA, have the EA edit the vCal file, copy that edited vCal file, copy it back to the boss, and have the boss delete the original calendar file then replace it with the edited vCal file. I run my own small company (25-30 employees), and there's no way I'd go through this crap daily - I ponied up $9 per month for an Office 365 account exactly for this, and I can then access my calendar frickin' everywhere; you'd blow that $9 per month in minutes at an hourly rate.
Take my advice, or you'll be wasting time and money. FWIW, MS lines all of this out on their web portal regarding compatibility and data exchange - anyone who offers a different solution in line with your posed scenario, well, doesn't know what they're writing about... (coulda/woulda/shoulda scenarios don't apply as they're not real...)