It would take some indication that the crime had occurred. If the victim testified, or there were signs that the rape occurred, then yes the admission is going to be damming.

In this case, in California court, the statement, while hearsay IS ADMISSIBLE. Statements made by one of the parties to the crime can be testified to by another in court.

What wouldn't be admissible is someone else saying that "Jill said Joe raped a girl" in an action against Joe.