Another awkward workplace situation--To tell or not tell another coworker about a "secret" meeting

Reading between the lines here - it seems like co-worked may not be the easiest person to work with,
To me, that is a perfectly valid reason to “exclude” her from a meeting - especially if she? has a reputation for being inflexible, not taking feedback or being easily offended…

True. I might ask boss “what’s up with the guest list” under those circumstances.

^ This, absolutely this. As a mid-senior level manager who schedules and attends a LOT of meetings, much thought goes in to inviting the tightest number of people necessary. If you are invited and think someone else should be, let me know. Please don’t forward it on your own. It could be that I’ll add them to the invite, it could be that I won’t. I want the right people there, but do not want to end up with a group so large that nothing gets accomplished.