Yabu.
You don't tell someone's boss something like this without their express permission, loads of reasons why she may not have wanted her boss to know. At the very least, a planned c-section implies possible medical issues with the pregnancy/birth that she may well not want work to know.
Or it could be as simple as the boss is usually ok but has weird views on c-sections she doesn't want to have to deal with.
With births - even MORE reason not to tell ANYONE without the parents of the child giving express permission.
A mistake I made a couple of times myself when much younger and before I wasn't a parent myself, I know better now - as a mother to 3 yourself you really should have the maturity and life experience to understand that.
"She is friendly with her boss and would constantly tell me how well they got on." Irrelevant - still not your place to say.
I've also been a "boss" and sometimes such information can put THEM in a difficult position with their boss/career wise. And it's a genie that CAN'T be put back in the bottle!
"she was telling the waitress she was going in for for a section and the date." Well she doesn't work for the waitress!
The waitress isn't potentially in a position to affect how her medical history is used at work, to harass her to return to work earlier than she may be ready to 'because you had that baby X weeks ago SURELY you're ready by now', the waitress isn't likely to tell other potentially less than supportive work colleagues, or higher up bosses, the waitress isn't in a position to hassle the new mum over paperwork etc
It's VERY much an employers market at the moment, there is a known increase in maternity and sex discrimination as a result and even if the boss you spoke to is an ok sort, perhaps their boss, who they're likely to now mention it to isn't?
I would NEVER mention anything like this to anybody's boss, you have NO IDEA the trouble you might be causing - again learned the hard way, incorrectly thinking bosses of mine were "ok" and I made the error of mentioning bumping into colleagues outside of work who were off sick, or in a "part of town" they wouldn't normally be because they were attending job interviews for rival companies etc... And unwittingly creating strife for colleagues, had that done TO me too!
"I wouldn't tell a someone's boss anything about them anyway to be fair." Exactly! You just don't do that.
You don't know what's going on in people's lives so its wise, if in ANY reasonable doubt, which I would argue should have been the case here, not to give certain people info they aren't already in possession of.
On one occasion with me it was a colleague had gone to the dentist and boss was pissed off that we didn't all go to her dentist brother! She could be a right arse about it!
"You come across as someone who cannot bear to be told they are wrong, or that there might be different ways of looking at things - which makes posting this question an odd decision." Totally agree with this.