I think his attitude isn't stupid. He enjoys what he does and moves on once he doesn't or has to.

I told him to stop being an ass

That's not how you convince someone. You should explain to him how this is going to heavily impact his reddit experience in the near future. When all the mods and power users are successfully alienated, reddit is going down the drain. It's them who carry the platform, not the lurkers.

Look mate, I get you, but I also get your friend, I used reddit pretty much the same as your friend, just lurked, didn't used any third party apps, but I saw things go to shit and decided I want none of that, and let's be real, the site offed itself a while ago, the people that moved to lemmy and kbin realized that, and where techie enough to adapt to the new environment. Some people on reddit will realize that sometime, some won't, and that's perfectly fine, they will either adapt to the fediverse, move back to where they were before or stay on the decaying site, but whatever they, and your friend do, there's no reason for hostility, especially between you and your friend, the deed had been done, and now we get to rebuild our communities in such a great way that we couldn't ever imagined over reddit.