I would have her give the parents the benefit of the doubt, seeing as how she is (by her own admission) largely ignorant of the details of this case and really doesn't know what she is talking about. But in this day and age we all have to have an opinion on a subject, no matter how little we know of the matter. Fortunately my daughter has no interest whatsoever in trying to convince others that her opinion on the McCanns should be listened to, nor spends any time whatsoever tweeting or posting about her doubts, so no harm done really.
She seem young and misguided, maybe in that rebelling phase lol.
I don't agree that children should give their parents the benefit of the doubt in all things ( Nazi Germany an example). However, if you explained that you believe the parents version of account and add the evidence/ proof for your argument it should bring her round. Or if you have no evidence you can explain the word believe to her a in believing in God with no evidence.
BTW if she is a teen good luck with that lol
I have no issue with people who believe in an abduction, I have issues with supporting trolls.