So you're just going to ignore the poor child too then?
What on earth is the OP supposed to do to help the other child? The school will be working behind the scenes and not telling her what they are doing. All she can concern herself with is what they are doing to protect her child from seeing/hearing things like that at school - the rest is between the child/parents/school. I'm sure she wishes the child well, but it's really out of her control, and it's not lacking in empathy to say so.
You can feel as badly as you like for the other child, but nothing anyone else says is going to help them, so there's no need to preface every remark with "of course I feel dreadful for the child, but how do I help mine...?", especially on anonymous social media? You can wish someone well, wish things weren't happening, wish things weren't like they are - but at the same time, also be concerned with what is happening that directly affects you, and you don't always have to acknowledge feeling sad for all the other problems around before you can mention yours.
I do think it's worth letting the school know that other children and witnessing the behaviour, and being affected by it, as they may be able to talk to them, to keep a closer eye on the situation, to stop children being alone together, etc.