I'm not really sure she meant the act of stripping down. It doesn't matter if you undress in front of your friends or walk naked into them on purpose, either way it's a big risk of ending your friendship forever, with most people.
if people end a friendship over something like that they were never friends in the first place
So I guess I've never had a friend?
No, I strongly disagree with your statement. In a society where nudity is a sexualized taboo, people might get all sorts of ideas and never want to talk to you again. For example, it doesn't matter how good friends you are, if your friend gets the idea that you're a sexual predator, you're no longer friends. You never know what might get in people's heads if you do something as unexpected as purposefully showing yourself naked without a warning. Incidental nudity such as quickly changing clothes is fine, you can always appologize if it causes a negative reaction, and your intention is very clear (to change clothes). But if you leave your clothes in another room and emerge naked, your intention is not clear at all and imagination fills the gaps.