First off, I want to tell u that u really did something wrong---------drinking too much! I wish this teached u that no one is to be trusted 100%
ok, now the real problem is that u can still get preg with only a slight amout of pre-ejeculation, but it's not likely at all though
and how sure are u that that "friend" did not really ejeculate inside u?
Let's say the baby is not ur fiance's, are u going to tell him? I think u should! I think u should even tell him now!! Asap! Why? Because how would he react if u told him after the baby is born? He would feel way worse!!!
I know that he might leave u becoz of this, but I suggest u to do that though. And when u do, really talk to him or communicate with him.
Tell him what really happaned and it was not ur fault. If he loves you truly and wants to spend the rest of his life with you, I believe that he will try to understand the situation.
Good luck to u and I am here for u if u need me