Girl problem...need advice.?

So I have this friend. Very cool guy, but to be honest, not the most attractive dude around. He has a really hot new girlfriend that he's been dating for a month now. Of course I should be happy for him but it bothers me. And I feel with good reason. It's just weird how the whole thing happened. It was in the cafeteria. My friend's just sittin there chillin. Random girl just walks up to him, introduces herself, and hands him her number. He calls, and then they procede to enter a relationship 4 days later. Not only that, but we're both 18, and she's 24. Of course he doesn't care, he's just happy to have a hot girl. But why would she walk up to a random guy she knows nothing about, and then enter a relationship with him 4 days later, knowing that she's 6 years older than him? IDK seems weird to me. I'm not trying to be a drag, but if you ask me this girl's up to somethin. What do you think?