Walking with friends, she often picks her nose. What does that mean?

It doesn't mean anything. Maybe it's her habit. If you really want to know the exact reason, then you can take the initiative to ask her. If you don't like her action, you can talk to her. It's not very elegant to go out like this. If you are in public, you can control yourself and say something euphemistically. After all, girls are thinner, and if you say it too bluntly, it will hurt you.