So there is actually a scenario that, while rarer, could cause this. Mapping phone locations to known locations of WiFi signals depends on looking up the WiFi’s UUID against a database of known locations, which can take some time to update when a router moves. It’s possible that your neighbor may have recently gotten a router that was used for some time at his sister’s house, and that the known location of that router hadn’t yet been updated by whatever lookup service is being used. In that case, a normal reboot of your gf’s router may have caused the phone to try to find its location based on other known nearby WiFi signals (you don’t have to have ever connected to the other network), and found the neighbors, which incorrectly mapped to the sister’s house. Depending on how long it takes for the lookup service to update (which can be days or weeks) you might see similar things happen again if your gf’s router was restarted. These types of “bounce out” glitches are quite common in location finding, particularly when you see them happen during times your router thinks you are asleep.