I wonder if by never saying anything negative about a absent parent that is not turning up for visits and being rather a poor parent to those dc - do the children get a distorted view of that parent and think that the resident parent thinks it is ok that they are treated like this as the resident parent doesn't ever say it is worng?
Could it be that more harm is done, that dc thinks it is normal and acceptable behaviour and then grow into an adult that thinks that it is ok.
Would it be far better to actually say well I dont think it is a very nice way to behave but you have to decide for yourself what and how your relationship is going to be like and leave it there?