I'd suggest installing Crammyboy's portal monitor and the FFS Lot debugger. With thee FFS Lot debugger... the user could try to reset all objects on the lot. If that doesn't fix it, they may have to turn on debug mode, reset all objects on the lot (maybe start with off-world objects?) and tediously and methodically inspect each error, to see what the object is. Any objects that might be spaceships or other vehicles you can probably safely delete. I would reset controllers and or portals rather than deleting them, since I'm not quite sure if all controllers will be re-instantiated. If this does not fix it, they may have to do the ole' move out and move back in trick to reset the lot.
One way to trouble shoot this, if it is indeed an invisible object ont he road, is to have their sims run up and down the road, trying to walk into whatever invisible object it is. If there is one, the sim
might show a thought bubble with an icon of the potentially invisible object. If it's a spaceship, then you can probably burninate it with the stuck object remover. If you get the generic footprints with an X mark blocking the path... it might be some other invisible thing. You could try burninating it, but if the portal monitor complains that it was a portal that was deleted, then the person may have to quit/exit without saving and/or do the move out/move in.
Ste