I'm hardly Mr. Experienced, but I've been smoking for a couple of years now, so I know the score, but I couldn't deal with this situation at all.
We had a huge meal and get stoned on the way to my house, he smokes a majority of the joint and I don't mind at all, I grab what's left of it and toke away. Later on, we went out to the supermarket, we got stoned on the way there and back again, he smokes a good amount of both and I ask him why he was so reluctant to pass that shit. He tells me that I've always accused him of being a lightweight and that he was trying to get higher than he usually gets because he claims I get higher than him. I think what the Hell, as long as we're all stoned right?
So we get to my house and he tells me he wants to lie on the floor because he feels weird, so I tell him to knock himself out and he lies on my floor. I'm watching some kick-ass documentary and am pretty much zoned out.
Then the worst thing ever happens, he tells me he feels 'too high' and then proceeds to vomit on my carpet and runs to the bathroom.
It really killed the vibe.