I wouldn't really have said he was being rude, he was sniffing her butt which is what dogs do to get an idea of the other dog?

And I get that! That's why I was guiding him away but from the very off, the other owner was very protective and immediately annoyed by the fact that her dog stopped for her butt to be sniffed.

And I fully agree that dogs shouldn't snap at each other but they have means and ways of communicating to each other that they're not happy with a situation which don't involve snapping - vocalising, moving away... That's why I said I don't just want him to learn from humans because he also needs to learn how to stick up for himself c: