You can't use water to float him out for at least two reasons: when sulfur dioxide gas mixes with water, it creates sulfuric acid; and the same fissures that let the gas in will probably let the water out.

If the air mattress is the kind that you would actuallly pump air into, fill it up. Turn the tent into a sling with the rope and drop it down, telling the man to crawl into it. Then drop the air mattress to him and tell him to inhale from the air mattress and exhale outside it.

But since the guy is twice your weight, I have no idea how to get him out of the hole by yourself. Running back down the trail, screaming, could be an option. <img src="/images/graemlins/crazy.gif" alt="" />

Sue