Weed doesn't cause cancer, perhaps you are thinking of cigarettes
How can it not increase the chance of cancer (even if to a lesser degree) if you smoked as many joints as some people smoke cigarettes per day (of course you'd have other problems if you tried that)?
The person you're replying too is clearly just spreading FUD and probably has never partaken in their live - sad!
We know that just inhaling smoke from a wood fueled stove will increase lung cancer risk. What would be unique with Marijuana that makes their imperfectly burned plant particulates more safe than any other imperfectly burned plant matter?
The CDC view is also that all smoke from burning things is harmful to the lungs and can cause cancer, regardless of what is being burned. People with lung damage is strongly recommended to avoid getting into contact with smoke, including smoking Marijuana.