His evidence is empirical - Apparently he gets pretty high with friends and shit talks - but when when the search started to suggest some pretty way out things along the same lines, he landed that their conversations weren't private any more.
So I have an understanding of how much tracking is going on so I pressed him on that. But he assured me it was stuff he would not even bother to look up in a clearer mindset and of course smoking recreationally for a very long time knows not to go near some tools that could land himself trouble or awkward explanations. That's probably true he says a lot of stuff that a half decent search would put him straight. In the end I just figured loose permissions of one of the many apps he's installed and that's how they (the app) make their money, selling illegally obtained data to more legal sources.
Permissions are the problem with android phones - there needs to be a specific install route for users, one that the app starts asking for things it should not need have access to, the installer refuses to install and suggests the user look for something better. Camera apps for example really don't need access to communication channels, if it's updates it's need, it can ask - one time access.