Although there is a large gap between progressives and traditionalists in terms of whether they check the "I believe in human-caused global warming," box on a survey, there is far less of a gap in terms of behaviors. I know plenty of self-proclaimed progressives who drive SUVs when it's not necessary, live in huge houses, and of course almost all of them are non-vegan, etc., and I know conservatives who are very conscientious about their environment, grow a lot of their own food, etc. Maybe it's different here in Montana, but being good stewards does not depend entirely on whether or not you believe in human-caused global warming. Even in this very red state, we're repairing grasslands, streambeds, dealing with a hundred years of damage from mining, vermiculite (asbestos) mines, etc. If you ask people why, they don't say "because of global warming!" they say "because we need to take better care of the environment." Although there are plenty to would deny global warming, there are none to think we should pour more chemicals in our rivers.
So, for your friend, maybe you can just focus on taking good care of the planet? I have conservatives in my family who would deny human-caused global warming but are more than willing to recycle their cans, plant native plants in their gardens, and even one who is approaching veganism (cautiously, but more fully than all but one of the progressives in my family). So, taking good care of the planet is another way to approach the issue rather than trying to convince someone to change their beliefs (which is essentially impossible, or so close to it that it can be considered so).
I was surprised to see this in the NYT recently:
Opinion | The End of Meat Is Here
It makes the point that there is a huge inconsistency in support of progressive policies and eating meat. I am going to send it to all the non-vegan progressives in my family.