Thank you for your question. I am very sorry for your daughter's situation, but hope my answer helps. The good thing in France is that women's rights are extremely well regarded (maybe one of the best countries in the world from this point of view), and from the point she will start official proceedings, she will be protected by the law.
The steps to take are as follows:
1. It is important to have informal help. If there are no local friends, then the best would be to look on this site (http://www.stop-violences-femmes.gouv.fr/) for the local women's protection association. They provide advice, shelter, good listeners, immediate financial aid if needed, as well as knowing the local social workers who will be essential in the resolution of the case;
2. Then she should also contact a local attorney where she lives, specialized in family law. Going through the divorce custody and process can be done alone, but I would not recommend it, because of the bureaucracy and the psychological pressure that her husband will certainly place over her. It is better to have a qualified legal help;
3. Based on the situation you have described, she has several rights:
-> to obtain a divorce;
-> to obtain the restitution of expenses made by her husband without her will;
-> to obtain compensation for the pain and suffering he has caused;
-> to obtain main custody (he would however keep some rights to major decisions regarding the child, unless he has been abusive towards the child).
Her citizenship status would not be affected in any way by this.
4. Even from the moment she files the claim, she can also ask the court to forbid temporarily the husband's access to their home (not respecting such an order is a crime), if she can prove (mostly with witnesses) that he is abusive / threatening.
One reason to appeal to the women's aid association is also to have the child under constant supervision, so that the husband cannot kidnap the child and threaten her like that. Bringing back kidnapped children, especially from abroad, is extremely hard (I know of countless sad cases). It is also very important that she places a Judicial Opposition to Travel (information here: http://circulaires.legifrance.gouv.fr/pdf/2012/11/cir_36117.pdf) for her child, so that the child would be automatically stopped at the border if the father tries to leave the country without the mother's consent.
I hope my answer was useful and look forward to your rating, which is essential to my activity.
Dr I L Vlad