You can't delete it.

Outlook needs a local PST for storing everything other than the email. IMAP doesn't support contacts or calendars, so that information gets stored in a personal PST file.

I would have gone through the same process you did, though I would have copied the emails, closed the old POP PST and stored it somewhere in the Documents folder so I'd have a backup. Those transfers to IMAP can be tricky and it's good to have a backup.

Also, going forward, your coworker should regularly back up those emails from the IMAP account. I've had occasions where my clients didn't have access to an IMAP account anymore, but the OST (file format used for IMAP and Exchange accounts) was still on their computer. I can't stress what a PITA it is to recover data from an OST file. It can't simply be loaded into Outlook like a PST file can. You can only convert it, and I've only found expensive third party conversion applications. I eventually bit the bullet and paid almost $100 for an OST converter.

*edit*
Just a little mini rant here in regards to the original question: Outlook 2016 has this obnoxious bug. As I've explained, IMAP doesn't support contacts or calendar so a regular PST file is created to handle that stuff. Now, some of my clients like to use that calendar "peek" feature that embeds their upcoming appointments on the right-hand side of the Mail view in 2016. For some bizarre reason, Microsoft has not made it possible to populate that peek window with anything other than the default calendar. But if your IMAP account is the default mail account, that means the unused calendar associated with that account is your default calendar. So that peek window is useless. I have no idea why they didn't catch this.