If a friend is posting a person's location, they likely post other information about that person. The aggregate information could allow the assembly of a much more complete picture of a person than is possible by observing a person at a random juice bar.

For example: are they married? Do they live alone? Who are some of their friends? Where do they work? What do they do at work? Have they recently lost/found/experienced something interesting? These are the kinds of details that can enable social engineering in digital and physical spaces. It's likely easier to lure a person into a dangerous situation if you reference minute details from their personal life.

Sometimes the threat model is "has $10k in a bank account" or "is young & attractive" or "practices a religion I don't like" or "it's Tuesday."