I think it was rape because as soon as she found out who she was having sex with, she said it was rape.
Ie. she hadn't consented, and wouldn't have consented to have sex with him, and no consent means rape.
Big, if you got drunk, laydown in bed with your partner, and just as you were sleepy and relaxed, she popped to the toilet for a condom, but instead, someone came back and shoved a lubed cucumber up your arse, that would be wrong wouldn't it?
Just because you'd been planning to have sex with your partner, doesn't mean that some other person can come and have sex with you - even if you really enjoy cucumbers up the arse, you only consented to your partner doing it, not some random stranger.