Approach him casually about it at first; lay the blame on the dealer and say you just realized you only got a half. Maybe he in fact didn't take a half and will be as surprised as you.

But if he did take a half, just tell him you didn't agree to that. That's a major ripoff. "Hooking someone up" is not worth a half ounce of weed. It's worth a couple smokeouts and maybe $10, but if he really is your friend I would think he would be willing to hook you up just because he's your friend.