'I told her that she was deciding something without knowing about it'.
Can I say 'I told her that she was deciding something not knowing about it'.
I guess both sentences are correct, aren't they?
I suppose so. They're both awkward. But in the right context, she'd understand what you were saying. I assume you mean that she was making a decision without really having enough relevant information about the matter.
Grammatically, the first is much simpler and more natural.