The idea of a rape sounds generally interesting but I wouldnt like it at all in the movie. It doesnt make any sense. Her death lasted only 5 seconds so I doubt theres any room for rape. Besides, with what (no penis) and for what? The creature is asexual and would have no point in doing anything sexual since its not how it reproducts. 5 seconds off scream and shes dead. In those 5 seconds he grabbed her and killed her. So the sex thing could only happen in about 2 seconds
The explanation I take is what we see on screen which is the tail closing in. Since it was originally Brett's death, I take what happened with the tail next as a canon explanation
What's certain is that he killed her fast. Because it sounded like she was choking Im guessing he ripped her open or something similar to this effect.