Hello Friend, Appreciate your interest to seek help from this forum. I can understand your suitation. Yes, it's not good to be in such negative mood when nursing a baby.
She may need a professional counselling support to overcome from this grief, else it can become deep rooted emotional disturbance. Choose online consultation as it will easy or convenient in her suitation.
Next Steps
Consult a psychologists directly or online. Feel free to reach out to me online through Practo app for further detailed evaluation and suitable recommendation. There are few sleeping techniques you can help her with to override the thought. Be positive. She will be alright soon.