I think you and Mystery Machine have a good point, but I do know a lot more about their home life than what I mentioned in my OP, and I think NHdisneylover hit the nail on the head. The kids do protect their mom. They squeeze in and out of the door, too. They make excuses for her behavior, and lately the older brother does most of the driving. I didn't mention this before, but I often smell alcohol on her breath and I believe that is why the son is driving her around.
Her mom can be very nice and fun at times, but the slightest thing will set her off and she can be extremely mean, too. I know they only get frozen dinners, because the girl complains about it and loves to eat over at our house. They watch a lot movies together, but at times her mom also makes her play outside for hours and won't let her back in even to go to the bathroom, and even in wet, cold weather.
I have heard the mom yell at the girl so loudly that I can hear her voice from across the room (coming out of the girl's cell phone, and it's not on speaker), over such minor things as being "late" home from a party that ended at 5:00, yet it's only 5:00 right then, or for not telling her she was coming over to our house that day after school, when she and I had a conversation about it the day before where she gave her permission.
I have seen the mom step on the foot of another child and just stand there, looking off into space, not moving, while the child is crying, tapping her arm and saying "Mrs XXXX, you're on my foot!" She snapped out of it when I came over and she apologized to the girl, but it was like she was in a trance.
If I do make the call, it will not be in haste. I've been thinking of this for months, and am not rushing into anything. I do have a lot more information than what I've posted, I was just trying to put out a few objective facts, because many of the specific stories can be so subjective, and any one story alone can be excused, it's the long-term pattern that concerns me.
I honestly believe the mom is either drunk or on drugs most of the time, or she has a serious mental problem. Her behavior is really that erratic. It's not so much about the dirty house as it is about the state of mind the mother is in that she allows it to get that way, and of course, the possible health hazard.