Replying here as Reddit will not let me reply to any top post on any Reddit (new account - actually, first post of first account). Anyway....
For the OP, I agree 100% with the "let him be" replies. I think you'll listen to them, but in case you won't, here are your strategies:
A) OUT HIM. In this case, you risk a big rift in your friendship. And while you are taking the time to get that back to 'good', know that every other teenage girl in your town will suddenly see him as a hot commodity. He will be the experienced boy who 26-year-olds want. You don't want this to happen.
B) SABOTAGE. By your age and the tone of your post, I'm going to guess you're not that experienced in head-games. You will be discovered, probably by your friend and certainly by the 26-year-old. Your friendship will certainly not survive. He will see you as the manipulative, crazy friend.
C) WAIT IT OUT. Virtually all 17-26 pairings die away after a little while. And when it does, consider that he will be a more experienced lover. The risk, of course, is that he may no longer be interested in teenage drama - but if you play it cool now, you will mitigate that some.