It's important that she understand he is not in control of his emotions; it is not his fault or his choice. Second, she should know that any time she speaks to him about what he hasn't done, she is triggering his amygdala and his flight response. She should avoid confrontation and find alternate solutions because the alternative (getting him to perform) is unwinnable. Positive reinforcement helps.
Lastly, she should look into enrolling him in EMDR therapy to assist with his PTSD. As a last resort, she should take MDMA with him as studies increasingly show breakthroughs.
Ultimately, none of this is her fault, either. She cannot "fix" him. She must accept him as he is, find a communication style that works, and be patient, if not for her then her son. Losing his father will create trauma of its own, if it exceeds the disfunctional relationship issues.