You are on the right track. Since you are able to realize that you would not have met her (and she is the best thing that has ever happened to you) had she not been his friend, and you are able to make that connection and feel thankful to the friend for the introduction, you appear fine. The obsessive fantasies will probably leave your mind sooner if you do not try to stop them but go with the flow. Do not try to get those images out of your head as it would likely exacerbate rather than help matters.

That you realize that she did not just appear in your life out of nowhere but came through the friend is really great. That she realizes it is also great.

The only slightly inconsistent thing in your account is that you yourself asked the friend if she was an "easy" girl but then went on to accuse the friend of being a "player" and treating women like ****. It does not compute. Plus, why is sleeping with a friend bad treatment of the friend? Look, he introduced her to you as a woman who is "great" (quoting OP) - how did that amount to his treating her like ****??

I wish you luck and applaud you for thanking her for the disclosure. She also appears really smart and thoughtful in that she realized that telling you face-to-face was better than waiting until you find out from third parties.

That you are thankful for the fact that she delayed the disclosure since otherwise you would not have fallen for her, and are appreciative, is also great.

It might help if you ask her to actually remember the details of the encounters if she can (she might not be able to, since she was drunk, might have repressed the memory, etc.). If you know exactly what happened, you might stop imagining things that could have happened. Imagination can be very powerful and fantasies can run wild, whereas in reality (as you aptly pointed out) it was probably really mediocre sex between drunk buddies. Nothing to write home about.