It's not their data to share.

It's data entrusted to that specific person with the social contract to not share that data (assuming no permission for sharing was given). I would argue the same way if someone posted my full address online for example, you're breaking the social contract by sharing data I entrusted you with.

Perhaps they didn't tell their friend about that one abusive ex or an affair their having. Additionally in this case it is telling people that when this person is in a certain location they are having a bad day, possibly opening them up to harassment a scam or manipulation.

There are a lot of people that will use that information to their advantage, even if it is just for trolling.

For me, if I found out about a post like this I would stop any contact with the person effective immediately after informing them of not appreciating the post.