I've played it where there are a few cards that have an exception and those have been in the dog and his vet. The mortar says it damages everything on a tile which I read as enemy and players.
That certainly may be true, and Mike Gnade will have to confirm that, be it the case.
Thus far, there is no precedence set by any other cards that would lead me to believe that one can attack other players in any capacity (purposeful or otherwise). I think "everything" here was likely used as a shortcut to writing "every monster" or "each monster".