Flying a flag upside down is traditionally understood as a signal of distress or emergency. In the context of the United States, the Flag Code states that the flag should only be flown upside down in situations of extreme danger to life or property.
If your neighbor is flying their flag upside down, it might indicate that they are in distress or facing a serious problem. However, there are other interpretations as well, including acts of protest or political statements, depending on the context and the individual’s intentions.
If you feel comfortable doing so, it might be a good idea to check in with your neighbor to see if they need assistance or if there is a specific reason for this display.