What you can do
Abusive relationships are awful, and unfortunately helping someone who is in one is not as easy as it should be. There are a few levels of help that you can provide which I will go through.
A very important thing to start off with is to be supportive. Abusers often try to isolate their victims so that they feel helpless. It is important to be supportive and help her know that she is not alone. There are a number of ways that you can be supportive of her. The US department of Health provides a good list of things you can do.
Set up a time to talk. Try to make sure you have privacy and won’t be distracted or interrupted. Visit your loved one in person if possible.
Let her know you’re concerned about her safety. Be honest. Tell her about times when you were worried about her. Help her see that abuse is wrong. She may not respond right away, or she may even get defensive or deny the abuse. Let her know you want to help and will be there to support her in whatever decision she makes.
Be supportive. Listen to your loved one. Keep in mind that it may be very hard for her to talk about the abuse. Tell her that she is not alone and that people want to help. If she wants help, ask her what you can do.
Offer specific help. You might say you are willing to just listen, to help her with child care, or to provide transportation, for example.
The Australian government has a good list of things to avoid doing as well.
Don’t blame her for the abuse or ask questions like ‘what did you do for him to treat you like that?’ or ‘why do you put up with it?’, or ‘how can you still be in love with him?’ These questions suggest that it is somehow her fault.
Don’t be critical if she says she still loves her partner, or if she leaves but then returns to the relationship. Leaving an abusive partner takes time, and your support is really important.
Don’t criticise her partner. Criticise the abusive behaviour and let her know that no-one has the right to abuse her (for example, say ‘your partner shouldn’t treat you like that’). Criticism of her partner is only likely to make her want to defend him or her.
Don’t give advice, or tell her what you would do. This will only reduce her confidence to make her own decisions. Listen to her and give her information, not advice.
Don’t pressure her to leave or try to make decisions on her behalf. Focus on listening and supporting her to make her own decisions. She knows her own situation best.
Ultimately, you are likely not trained to help get someone away from abuse, but you can get her in contact with someone who is. Many countries have national abuse hotlines that operate 24/7 to help victims. As I know you are in France, this site provides a list of helpful resources that you can provide her.
When you talk to her
When you are talking to her, be sympathetic. When she posts about the things that are happening, you can respond with a sympathetic statement and then an offer to talk further.
Her: My husband did [explanation of abusive behavior]
You: That's awful, I'm sorry that you had to go through that. If you ever need or want to talk about this, feel free to message me and I'd be happy to listen.
This would be a good place to mention the resources. You can let her know that if you aren't available at that exact moment, that she can call one of the helplines and there will be someone there she can talk to.