It's possible that the phrase is:
* A local or regional expression: Meaning might vary depending on the speaker's location and cultural context.
* A newly coined phrase: Someone might have recently used it, and it hasn't gained widespread recognition.
* A misheard or misinterpreted phrase: The actual phrase might be something else entirely.
To understand what "sidewalk friends" might mean, you'd need more context. For example, where did you hear it? What was the speaker talking about?
Let me know if you can provide more information, and I might be able to give you a more accurate interpretation.