My opinion ...
Yes you had every right to get mad or upset. I mean crap!! There is beer in one of your most prized possessions!!!! I think anyone with a pulse would have the same reaction.
At the same time, it was an accident so you can't really get upset at that person. Especially a "best friend". If he truly was a "best friend" he was probably putting a huge guilt trip on himself. Put yourself in his shoes. How would you feel if you potentially ruined your best friend's guitar?
And as everyone else said, you assumed the risk of bringing the guitar to the party. You had to see that there was a chance at this happening. That's on you.
I don't know the whole context of the story, but after the incident happened I am picturing the kid from back in the day that got upset and says "I am taking my ball and going home". As Dan said, that probably is what triggered your friend from feeling bad to getting defensive and angry.
As for your friend, if he was feeling no remorse for the whole situation (even if it was an accident) ... yeah I would be pretty ticked about that. And his whole guilt trip and wanting to fight you is ridiculous.