A Frape Whistle is any act that informs someone that they are in the process of being fraped. To blow your frape whistle is to alert someone that their profile is being tampered with. This term is most commonly used either in a threatening manner while someone is being fraped or with a regretful tone after the event.
We were fraping Mark's profile but Julie blew her frape whistle before we could finish.
Oh that is the worst frape I have ever seen, someone really should have blown their frape whistle.