Hey Robert. Thanks for reaching out. I hope we can help.
Yes, it sounds like your girlfriend was sexually assaulted and/or raped. I don't know which country you're from , but the United States Department of Justice defines rape as “The penetration, no matter how slight, of the vagina or anus with any body part or object, or oral penetration by a sex organ of another person, without the consent of the victim.” So, if you are in the United States, yes, your girlfriend's experience falls within the definition of rape.
Your girlfriend could legally not consent to any sex acts if she was drunk. Alcohol negates consent. Given the fact that her friend later said he intentionally meant to get her drunk and planned to "seduce" her makes it very apparently sexual assault.
I am glad you were willing to listen to your girlfriend open up and give more details about what happened. While one can't know exactly what she's feeling, victims of sexual assault and rape often feel confused, scared, worthless, and like what happened is their fault. I hope you will be there to support her going forward. She may want to seek therapy, or, if she's religious, guidance from a spiritual figure. I hope you support her, if that's something she decides.
I do want to say that, though I imagine you have many feelings about what happened, please do not push her to prosecute her friend, or report him to the authorities. You might want to suggest those are options, and if she chooses to go forward, support and aid her in that, but this kind of experience is very, very hard to go through. Reliving it for the authorities can be traumatizing all over again, and unfortunately, these crimes often don't really get prosecuted. Very few rapists actually experience legal consequences. So I know you might want to prosecute, but remember, that's her decision. The best thing you can do is to be there, listen, and show her you still care about her, because often these experiences make people think they are worthless and unlovable.
I hope this helped.
PM me if you need anything, okay? Take care of yourself, and your girlfriend.