“It seems that a girl told a priest in a confessional that a 64 year old man was raping her and the priest treated that as a confession?”
She was either seeking advice because she had no way to stop the rapist, or she might have been guilty of allowing the rape to happen. You can’t possibly know which it was.
A 12 year old guilty of “allowing” a rape? Listen to yourself.
While I can't agree with the part about a twelve year old allowing a rape (no capacity to give consent), there is a more logical possibility.
Children this age are ego centric and will often take blame upon themselves for things that we, as adults, know are beyond their control. For instance, thinking that parents divorced because they were "bad" kids.
If the perp groomed the child as most perps do, he would have easily been able to reinforce that erroneous thought she might have about being at fault.
If that was the case, she may have come forward to confess her mistaken belief that she was at fault, in which case it would be the confessor's religious duty to help her understand the fallacy of her thinking while directing her to the resources to make the report.
If this was the scenario that unfolded, then I suppose the priest could have assisted her to make the report.