So, I work with sex offenders as my day job. Most of them are nice guys who ended up making a poor decision with someone underage. Let me break down how these fine gentlemen got caught up in... THE OFFENSE CHAAAAAAAAAIN
1) The seemingly unimportant decision. In this step, you make a poor choice which seems perfectly innocent. The poor choice puts you in a situation of being in danger of doing something illegal. In this case, it would be choosing to hang out with your friend, while saying you're nothing is going to happen.
2) The temptation to offend. In this step, you are tempted to commit the illegal act. She's well spoken and gives you an intellectual boner? Well she's probably also young and innocent enough to give you a real boner. (This may not happen, but if it does, you're now in danger of..."
3) Rationalizing. You minimize the harm that sex with a minor would do, or make excuses, or think up convincing lies if questioned... and convince yourself that the illegal act would be a good idea. See rationalizations for more details:
http://en.wikipedia.org/wiki/Rationalization_(making_excuses)#See_also.
4) Giving in. What I like to call the "fuck-its," you decide to go through with the act. You have amazing sex with an assenting and attractive partner.
5) "Oh shit, I'm going to jail." Congratulations! You're now a registered sex offender! I'll be seeing you in my group therapy sessions, where you can learn the difference between an assenting partner and a consenting partner.
TL;DR:
If you think you're going to be tempted to have sex with her, end everything immediately and never look back.