If she dials 911 about a bomb, the cops would (depending on the location) be there in minutes, right?
Assuming basic competence of jury, lawyers, and judge....
If there's a credible argument that he was going to use the bomb imminently, I wouldn't expect a jury to convict her for murder. While vigilante justice is frowned upon, don't most states recognize the need for citizens to use force to stop crimes? Similarly, if there's credible argument that calling the cops would put herself or her child in danger - say, she *just discovered the bomb*, and had a confrontation with him - she probably has a self-defense defense. As a long-term sufferer of abuse, she might have made it with a temporary insanity defense as well...
If there *wasn't* credible argument that he was going out imminently - if the police response time would have been good enough, if she could go next door to make that call - then a murder charge seems like the way it would go.