People disbelieve victims of sexual violence for every reason under the sun: based on how we were dressed, the way we walk, the way we talk, what our color is, what our social class is, what our gender is (this is a biggie with male survivors), what our sexual orientation is, if we've been sexually active before or not, if we wear our hair this way, if we have this size of hips, breasts, thighs, penises, if we're disabled, if we've enjoyed consensual sex before or not...you name it, it's been used as a way to rationalize sexual violence. But there's not a one of those things which justifies it or ever has. The fact that your friend flirts or is sometimes seductive when she's feeling that -- if she is at all, since often reputations aren't even based in truths or reality -- doesn't make her victimization any less valid than it would for someone who isn't flirty. Flirting is not consent, nor is a person being flirted with somehow being given permission or the right to so anything or everything sexual they want to do to that person.
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Too, a person not protesting -- even though in this case your friend did protest -- still isn't consent. Consent to sex isn't just the absence of a "no." Consent is an enthusiastic, strong yes: an expressed wish, physically and verbally, to share a mutually felt desire. The desire for sex is not passive or weak, so it doesn't make a lot of sense for anyone to suggest that the expression of that desire would or should be. In fact, if you want one way to spot someone who is a potential rapist or a rape enabler, pay attention when you hear a person say that not saying no to sex is the same as saying yes. And know that were the shoe on the other foot, and were some man forcing himself on them, they certainly would not say their lack of protest was equivalent to consent.
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You can make clear that there isn't any grey area here. She said no, this guy raped her, purposefully dismissing her no to get what he wanted against her will. She was violated and abused by this person. That's not murky: it's incredibly clear-cut. It can be helpful to try and let go of terms like "taking advantage," too. He didn't take advantage. He raped. Using phrases and words that make something violent, terrifying, abusive and harmful seem less so or benign not only can feed into enabling those things, it can make it a lot tougher for victims of abuses to turn into survivors, and put the blame where it belongs, calling a spade a spade. A lot of those phrases we hear -- like "taking advtantage," "grey rape," or calling any kind of rape sex -- come from a cultural desire to deny or dismiss abuses; from the desire of those who abuse or enable abuse to shirk responsibility.
Absolutely: despite the advent of terms like "gray rape" popping up in mainstream publications and gaining social currency, the term is an oxymoron. By definition, rape cannot be gray -- either consent was obtained or it was not. Because of the dangerous myths that the term perpetuates, it's imperative that we remove "gray rape" from our vocabularies, and call out other people when they use it.
Corinna's entire response is a long one but it's definitely worth your time. You'll be doing yourself a favor to read it all and pass it along.