You can expel the gasses from your lungs regardless of their weight, you simply die because you're not giving your body enough oxygen, and usually the gasses people huff are poisonous, so if you don't die from suffocation, the gasses themselves will kill you.
I would never believe "facts" given on a public show like that, they don't cite sources and could be completely wrong. I've talked with many people that do interventions, and even though they help people's addictions, they sure don't know what they're talking about half the time.
I'd say the best harm reduction would simply be don't huff gasses and chemicals, just get better drugs. If someone wants to get high that badly, smoke some weed.
And I'd have to agree that meth is worse than heroin. I've done heroin off and on before, and while addiction was hard to avoid, it had no ill effects on my body if I kept my usage in control. Meth is pretty much impossible to use without running into problems, no matter how sparingly you use it.
Edit: Me and my bud tried using a meth pipe to smoke heroin... it's not practical, the inside of the pipe gets all blacked up and you can' tell how much you're smoking/ just burning and wasting. If you want to smoke heroin, use aluminum foil and "chase the dragon."