the plumber...
so, what the plumber eventually determined (with the great scientific method of taking a bucket with some water on it and pouring it on the floor upstairs) is that there is an actual hole in the floor beside their toilet through which all water happily flows down because their floor is tilted. when they take their soaking wet babies out of the tub, water goes everywhere and, eventually, makes its way down. why they don't use a bathmat or at the very least a towel on the floor is beyond me. so, um, they're gonna get that fixed, and they're out $90 for the privilege of being told this useful tidbit of information.