Ok now let me put this simply...
I like this girl lets call her Brenda and my best friends name is Bob now me and Brenda have a LOT in common and always joke around about stuff "friends" don't joke around about. I know this and have been building a relationship with her for 2 years and around the time I'm about to ask her out, Bob tells her friends that he likes her... >expletive removed< so Bob tells me about this unaware I like her too and I do probably the dumbest thing ever and aid him in his efforts because all his life he's been a wimp and could not ask a girl out.So Bob asks out Brenda and live happily ever after... right, well no once again I do something just plain stupid and tell him I like his girlfriend which if you're familiar with the bro code, absolutely prohibits this.And now I know why because my friend HATES me.
What do I do?

How old are you guys?
You were a wimp too, by not asking her out a long time ago, then made it worse.
APOLOGIZE. Don't tell him the whole story, just say you are sorry, and won't do anything to ruin their relationship.
If he's as wimpy as you say, be glad for him, and if you are as capable of getting girlfriends as you imply, then move on and find one.