The alcohol is no excuse. It lowers inhibitions, which just means that certain personality traits come out more readily. In vino veritas, and your truth is you are an asshole.
Unless you happen to think that guys who sleep with their 'best friend's' GFs are good guys. The fact that you did it doesn't make you different, it makes you one of those guys. Oh, she's an ass too, in case you were wondering.
Yes, you should tell him. Its the decent thing, to warn him his GF is a cheater. But you won't. Few guys who would do what you did certainly wouldn't have the balls to own up to it. It would be a remarkable act of maturity and would mean you actually take responsibility for your mistakes. That's okay, tho. Sooner or later it will come out. It always does.
Grow up. Good luck.
				
			 
			
		 
			
				
			
				 
			
			
				Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh