What should have been done?

When this girl was 6-7 She remembers going onto the computer and sometimes this boy wouldn’t close the tabs or clear the search history so I would see what he was watching. He would watch hardcore bdsm where the girls would be tied down, have marks and bruises on them and gagged while being penetrated usually in a dungeon. The videos would scare the girl and she didn’t understand what was going on. The boy that didn’t close the tabs was about 14 years old, his parents knew he would watch it but they didn’t stop him or do anything about it, he even got his own laptop which he shared with the girl and he continued to watch these videos and not close the tabs or clear the history so the little girl would see it too. His parents would even allow him to hang up posters of p***stars in his room and told the younger girl not to go in there for that reason. It ended when she was about 10 years old but she would be told not to watch those types of videos (which makes sense but why would they allow the boy to view it?) The boy going through puberty and having an Interest in that or possibly curious makes sense why he would watch it but he would also act it out on the girl which is why the sister couldn’t be in his room with him and most of the times they were separated due to it. I personally somewhat blame the parents for lousy parenting and not keeping the girl safe.