The mother picked a man that was crazy enough to kill her childs actual father at a police station to be around her child. Does she hold any blame for her own actions and choices?
The mother's boyfriend is responsible for the murder. While it is true that if the ex wasn't present, he couldn't have murdered anyone, this "blame" argument hinges on the assumption that she knew her boyfriend would murder someone in front of a police station.
She chose to bring someone she is dating to a drop off at a safe location and in hindsight, we can all see that it wasn't the best decision. The question is, did she think it was a poor decision prior to her husband being murdered in front of her daughter.