Parent-Child Boundaries

I am replying to my own question to further the information. I am thinking a lot about boundaries lately. A very beautiful woman I know has 4 beautiful children. She, the mother, is an alcoholic. Her ex-husband is raising the children, and has remarried. He and his new wife never say anything negative to the children about their alcoholic mother. They do say neutral or nice things such as, "We hope the best for her, and we will pray God will look out for her." I think this is very respectful.
My mother was left with four children to raise by herself after my father was caught sleeping with the babysitter on the couch on Christmas Eve. My mother basically walked in on them after working a long evening. Needless to say, it was not a good holiday, although I was too young to remember any of it. My dad was kicked out and never lived with the family again. I saw him once every couple of years.
Throughout my life my mother never said a negative word about him. She was neutral, never praising him or degrading him. I find that rather commendable which makes me feel that the parent-child relationship is rather sacred. Rather the parent is good or not, rather the child is successful in life or not there is something within the boundaries of that relationship that is honorable. What do all of you think?

So here is my question: What is the parent-child boundary?