YANBU. Whatever your reasons.
YABU if you expect thanks. YABU if you think it will force change. However, you have done all you can do.
And actually, YANBU to have done it because she went against your advice and went back... because really if she's adamant she wants to stay with him and SS turn up she may well convince them all is well and she was just ranting to a friend.
Been in a similar situation as your friend but without children involved, and it's a desparate, desolate place to be. Frustrating for anyone who cares. With children involved I'm sure there's a different spin on it, and consequences, but ultimately she's your mate and you want her safe... so please keep telling her that, encouraging her when she's feeling brave (have an action plan) and supporting her when she's not, and be prepared for her to change between those feelings in a blink. It's all you can do.
It all sounds so bleak, but trust me, if you are there (again and again and again) when she's feeling strong, and you have a plan, and she's is up for that plan, then you may be the key factor in her escape. I wouldn't be where I am now if it wasn't for a very patient mate who had a plan, and a 30 minute rage that made me follow it to the point of no return. But kids weren't involved, so hopefully someone can come on who's been there with children/teens who'll better advise you.
Unreasonable or not, you're a good friend, and I suspect it's impossible to put your own feelings aside when you know someone's folded and gone back to a bleak existence.