There is THC in bongwater, but not very much. If you dry out the bongwater, it's pretty much the same sh*t you'll get from scraping a pipe. And it's true that THC is not soluble in water, but that has nothing to do with the topic. As others have mentioned, there's no connection between what's in the water and what's in the smoke. And the water won't affect the smoke, no matter how clean or dirty it is. For example, some people think that using wine as bongwater will get you drunk or super-high - that's also a myth. If your friend doesn't realize this, he may be smoking a bit too much

Please pour that sh*t out and sterilize that bong. Make it your job to educate your foolish friend. I had to do the same thing with a friend of mine - he would not change the water for months and it was horrible. But I would clean his bong every time I went over and he eventually realized how much better it was. I like to clean my glass bong every 3 sessions and put it in the dishwasher every month or so and it makes every bowl taste better. I think dirty water actually makes the high worse because you're only thinking about the disease-infested cesspool of filth you're putting lips on.