My husband had an affair with a colleague many years ago. At the time he was so distracted, so unlike his usual self and so less able to focus on our kids that one of my biggest worries during the brief time we were separated was that something would happen to harm the kids while they were out with him and her. He was so caught up in what was happening, it was so romantic to him, that he was more like a lovesick teenager and less like a mature man. He recognises, in retrospect, that the kids were a lesser priority to him during the period of his affair than they ever had been, or have been since.
I think you can be of the opinion that the father's relationship most likely contributed to this happening without that meaning you're a judgemental, man hating, moralistic, prurient harridan. I've witnessed the impact a new romantic relationship can have on a person, and the way that it can compromise their ability to focus their attention on the world outside the relationship bubble.
I feel enormous empathy for this man, his wife, his girlfriend, the children, granparents and everyone touched by the horror of losing Rebecca. I still think that the new relationship is a factor, and would be very surprised if the father didn't feel the same way. It's likely to magnify his guilt and be something he beats himself up over.