I do not believe this co-worker breached any confidentiality laws because everyone has read-only access to boss' calendar and could have stumbled upon this information themselves and some probably have.

I, however, plan on keeping the information to myself (except here where we are pretty much anonymous, because I needed to blurt it to somebody before I burst).

I hope you are right that it is nothing to worry about and it's not a job interview for him.  I am keeping my fingers crossed.