Rape is a very strong word. If it is actually true, then Danielle is doing other women a disservice by leaving the guy unpunished to do it again. You or Christine might be the next victim. The only proven solution to a rapist is jail.
This could also be a case of a jerk insulting a girl after a hot session (he sounds like the sort who makes all sorts of promises before... and afterward tosses her off like a dirty diaper), and hurt feelings being expressed in a rather dramatic manner.
I am not trying to make excuses for a bonafide rapist, just pointing out that you really don't know one way or another. Rape is a serious crime with serious jail time, and a seriously wrecked life after one gets out of jail... convicted felons don't have much of a career path, and sex offenders are registered as such for life. Be very careful repeating accusations of a felony crime, in the absence of any proof. You'll be opening a can of worms you really don't want to be near. Might even get an expensive libel lawsuit slapped on you.
If it's true, Danielle should take action to save other women. If it's not true, then Danielle should learn to spot jerks before she gets involved with them.
The only thing you know as fact is that this guy acts in a manner that is disrespectful to Alice, and will probably continue to do so. I've been in situations like this before - seeing this sort of thing going on. Seen men acting this way towards women, and women acting this way towards men.
The best course of action I've found is to be quietly supportive of my friend (I don't count anyone who acts in a cavalier manner towards their significant other as a friend) but not mention the activity directly. Eventually, they will figure it out. If you directly intervene in someone else's romantic situation, both of them may turn on you, regardless of how good your intentions may have been.
I had a friend's wife hit on me, in a not too subtle manner. I politely declined, saying I was very flattered, and that I was going to forget she made the suggestion. And made sure I wasn't alone with her again. I did not want to get in the middle of someone else's personal problems, what with high emotions flying everywhere. Eventually, she got caught cheating on him, several people (myself included) endorsed his idea of divorcing her, and they split for good.
Later on, he did ask me if she ever came on to me, and I said - yeah, sort of, but I didn't know what to do... very awkward situation. He said I probably did the right thing in declining and saying nothing, ruefully admitting that some lessons you just have to learn for yourself.