Have you ever heard someone hear about one rape case that was unfounded or thrown out because of a false accusation and then generalize to all rape cases? Overgeneralization, jumping to conclusions, and blaming are among the cognitive distortions that people often call upon as a way to dismiss violence against women.
So what can you do: Speak up and point to the evidence. Point out what evidence there is about reporting rape. (1) The majority of rape victims do not report being attacked to the police (NIJ). (2) Research shows that false reporting of sexual assault ranges from 2-8%. It means it happens but it is not typical (NSVRC).
Even if the person you are talking to does not change his/her mind, someone else may hear what you are saying and you have had a chance to practice speaking up about injustice.
NSVRC.National Sexual Violence Resource Center. 2016. False Reports: Moving Beyondthe Issue to Successfully Investigate and Prosecute Non-Stranger SexualAssault.