Lots of people struggle with boundaries; luckily there are many little sayings along the lines of "not my circus, not my monkey" that help people remember not to get worked up about someone else's problems.
It's a useful strategy to notice when you're getting too invested in other people's lives. It's really handy to be able to let go of how people perceive you or whether or not they like you. We have a strong need for belonging, and we can sometimes be easily guilted into taking on the burdens of others just to get a little taste of perceived connection. So, sure, if it's not your fault or responsibility, then it's not your problem.
There's a "Let Them" poem (by Cassie Phillips) that blew up then got somewhat plagiarized into a book deal for Mel Robbins. (I actually only clicked on that video because I somehow thought it was about the very funny comedian Mel Giedroyc! The thumbnail looked a bit like her, and, apparently, I didn't actually know her last name! But then I got hooked by the content.) Robbins' daughter used the phrase "let them" to talk her mom out of nosing in to her son's plans for prom. This is a great philosophy to shut down the busybodies in your life, but I can't imagine those two words flushed out into an entire book, and you could get this, and so much more, from books on Buddhism or Stoicism. I'm clearly not the target audience.
