Don't waste your limited time here on earth arguing with people you'll never convince. 
You could have pointed out that so far, his diet has caused the destruction of 20% of the Amazon rainforest and at least 20% more is expected to disappear in the next 15 years and all for the sake of cattle ranching. How many leaves is that?So someone on a forum told me this about veganism and the environment:
Him: "Animals take Oxygen and breathe out Carbon Dioxide which Humans can't use.
Plants take Carbon Dioxide and breathe out Oxygen which Humans can use.
By eating plants you are killing off our supply of Oxygen and causing an increased level of Carbon Dioxide production.
Vegans are destroying our planet"
Me: "Vegetarians use less resources like water, food, oil, contribute less CO2 to the atmosphere, and animals are not killed for their consumption.
Raising livestock contribute more to global warming than automobiles, and is the second leading cause of global warming behind industrial pollution.
Being a vegan for 1 year;
How many animals a vegan saves: 200~
How many lbs of meat a vegan didn't eat: 200~
Lbs of CO2 not released into the atmosphere: 1,609~
Also, we usually don't eat trees. Your claim is literally nonsense.."
Him: "How much oxygen is produced by a single leaf in an hour : 5mL == 120mL a day == 43.8L of oxygen a year from a single leaf. Humans requires about 50L of Oxygen an hour to survive. ( source =https://io9.gizmodo.com/5955071/how-many-plants-would-you-need-to-generate-oxygen-for-yourself-in-an-airlock ).
Every time you rip a leaf off a plant you're effectively making someone's life over the course of a year 1 hour shorter. How do you feel about this?"
Me: [Insert what I should say here]
Thats a great point and then you can pull in the fact that we are killing the oceans with acidification and toxins and yes, even killing the whales. I read once that the waste/feces from whales is vital to the health of the ocean because it fertilizes all the algae that you just mentioned Stuart. Kill off the whales and the algae begins to die off which means that all the fish and other creatures that depend on algae begins to die off too.I once heard that if we paved over the entire planet we would be OK when it comes to oxygen because of the algae that is in the oceans. It produces something to the tune of 75% of all of the oxygen that we breathe.
Check out this link: microbeworld.org/types-of-microbes/protista/algae (sorry I can't hotlink since I am too new)
His logic is flawed because he isn't accounting for all of the oxygen that is put back into the air at a greater rate or just as fast of a rate that it is removed. The ecosystem is naturally balanced.
......
You have some sort of personal problem apparently, since you can't evaluate from my statement that there are people on the Intertardz who who still think climate science is up for debate, and that there are equally opposing views on things like man made climate destruction, and there are still people who don't know that veganism is approved for all life stages by the World Health Organization. There are idiots on line who think Brietbart and Popular Mechanics are valid sources of scientific information, or who claim they aren't vegan, "because science" though they were a communications major with a C average who gleaned that information from Dr.Atkins, or paleo.'Sources: real science classes, not some website.'
And where do you think scientists and teachers are putting the information that they are teaching in their 'science classes'? You can even register and read archived Harvard, or Berkley or MIT classes.....online. Who would have thunk it? https://www.edx.org
Sorry, will follow your recommendation from here on in.stop being so adversarial. Both of you