I mean they were about to have a baby together before things went down, even if he was a bit insensitive about the aftermath, other than full on scorched earth and 'revenge' on him too, i imagine it'd be hard to turn ur feelings off for that (despite him being the one to move on fairly easily)
Losing a child can kill a relationship very easily. People struggle with the loss and feelings of guilt, doesn't matter if someone actually did something wrong or not. Staying together can be a daily reminder of what you lost, so many people don't.
I like that she doesn't see him negatively though, while it would be nice if learned the truth after everything is over, it's probably better if he doesn't. Imagine the level of guilt one would feel, finding out that not only was the accident caused by a third party, but you didn't know and the person you loved fought everything about it alone - ignoring the revenge for this - that could easily break him and his new relationship.