I think for me it matters not one jot what I think - this poor man will be beating himself up every day for what happened whether I think it's his fault or not. What's really sad is that these poor parents have got to deal with both the tragedy of losing their daughter and the feeling that the whole world is watching and judging them, their behaviour and their marriage.
I think it's really common for wives to feel a sense of shame when their husband cheats, especially if other people know about it - the old 'am I so awful he had to look elsewhere' feeling - imagine how much worse it must be to have that AND for the whole world to think that the man you loved was not only unfaithful but also negligent of your children. I'd want to crawl into a cave and hide.
I can't imagine how a marriage could come back from something on this scale, and I know if I was the wife I would find it really hard not to blame the husband and his affair for what happened, even if I accepted that it was an accident.
Our opinion is frankly irrelevant. Poor poor people.