To me it sounds like he pre-planned this.

"Do you know that cocksucker SOB pulled a fast one and switch the bags. I got a bag of fucking sage or some shit."

So he already brought something to switch it with, even before it happened. That's lame. It doesn't sound like the he was "in on it" with the dealer, as I don't see how the dealer could have benefitted. Afterall, your so-called "friend" got you to split the costs with him. Then he jacked the whole thing and gave you a bag of bullshit. So he got the ounce for half-price. Maybe I misinterpreted your post, though.

But why didn't you look at the bag and tell him, "this isn't weed"? And then demand real weed, because you were to get half of the ounce. I know this sounds bad, but I would have beat him up and taken half of it myself (if he didn't willingly give me my share after demanding it), not just give him a black eye and leave without the weed, LOL.