What are you supposed to say to someone who believes climate change is real but won't accept that human activity contributes to it?

Nothing, having arguments with people that lack basic critical thinking skills is a waste of time.
A simple google search can give you all the data and numbers and you can show it to them but it doesn't matter if they believe they know better than actual scientists.
There are people out there that believe the Earth is flat.

Met a director of my company in the UK that made a joke about vegans. I said, say what you want but they’re good for the climate! He went off on me about how climate change was fake. I tried my best to just move onto a different topic but he just kept going at me and ended it with: “you started it” … to which I said, “yes and let’s move on shall we?”

They’re not worth the effort.