Obviously sympathetic to the mothers position here. However, based on my limited understanding of how this stuff works, if content is being viewed on 'the dark web' it likely wouldn't be obvious to internet providers, nor would a kid who's savvy enough to be on the 'the dark web' in the first place be really prevented from finding a work around to any potential block in place.
I'm a parent. This stuff scares the crap out of me, but I'm not sure we can actually legislate this stuff out of existence.
Also, who would be held responsible for the child circumventing the restrictions? You can't hold the child responsible, so that would fall to the parent.
Is she accepting responsibility here for not better protecting her child? No, she's blaming others, and telling other people they should take responsibility.
Not that any of this would have made any difference here. These two murderers were absolutely demented, and keeping them off social media wouldn't have done much to prevent that.