Well, even if she does reject the dude and says she already likes someone else, Taira would only think it is that other dude in his dream that she likes since she seems to be practically ignoring him. On the other hand, like a few other series I've read, she might just agree to be the random dude's girlfriend since she has no reason to refuse. And, of course, the other option is that she would agree to become friends first and then she and random dude would spend all their free time together and Taira would get left in the dust.
Since she actually agreed to meet the dude, she must have known that he was going to confess. Or is she really that stupid? And her friend must have known, as well.