Cadging off your neighbour is bad form
Good form would include letting them know that they are exposed. Just tell them you happened to notice, along those lines. I can see several of my neighbors WiFi routers. One was unlocked and it was blindingly obvious which one it was because they used their last name. They have since been advised and it has been corrected.
Of course, I know their password because I set it up for them.... I kid, I kid.