But then all of his traffic goes through my internet. Unless he has a special high end router that can only route specific domains.

You dont need high end box, a old mini pc running pfsense/opnsense is enough.

High end, as in feature rich.

If you think most non tech people can just casually find an old pc, install and setup pfsense, then you are out of touch with reality. And they also would have to let it run all the time and have space for it. That's just not something realistic. It's not an option for my friends. Might also break chromecast.

Most non tech people just subcribe to netflix/hbo/disney and call it a day.

Correct, most do. Still leaves a significant portion that does not want to sub to multiple streaming services. Be that as it may, I have multiple friends who I share my jellyfin with. Giving them all access to my network is just not an option. It's not.