"Innocent until proven guilty" is never true, even in court. A defendant can given a "not guilty" verdict, but that is not the same as being innocent. "Not guilty" means the defendant is not directly responsible for whatever specific charges they are being tried for. "Innocent" means the defendant has no knowledge, awareness, involvement, or relation to the case whatsoever.
"Not guilty until proven innocent" is merely the the scientific/statistical model of null
hypothesis testing in a court of law, where your objective is to find evidence to disprove that a certain event happened by chance and are unaffected by the variables you are testing (aka, the "null hypothesis").
In Himiko's case, I don't think there's enough evidence to prove or disprove that she abandoned her friends or was directly responsible for them getting raped. However, I really don't think objective evidence matters in this case: if Himiko's friends are angry enough to want to blame her, they can condemn her to this island without having to be rational.