This happened to my dd, SIL left when baby was 2, because he didn't get enough attention. The only thing to do is be there for her, make no judgement but let the hurt filter through until the dust settles. When this happens and there was no reconciliation, after 2 years, my DD woke up and took control. At no time did any of the family make negative remarks, mainly for the sake of the child. It was never discussed but accepted that he was an selfish pig. Now 11 years on, he has not changed, never grow up. His son sees him every other weekend etc because DD did not want him to hero worship his father on the grounds he will see the real person eventually. So he is tolerated for the sake of our grandson. We are all polite but secretly hoping it will end someday but unfortunately this will not happen unless grandson wants it too.
It just makes my blood boil that such men can get away with abandoning their family and responsibilities, as to this date he has paid nothing for the up keep of his son, but that is discussion for another day.