Start a conversation with a companion character. Switch characters and attack this companion until they have 0hp. Switch back to the conversation and send the companion to camp. Go to camp, have 0hp companion rejoin party. Fast travel to a waypoint with companion(but don't use Leave Camp as it will restore companion's health and prevent exploit). Companion is immortal until having some health restored.
Saw a post where someone encountered a similar bug last patch. They encountered it more naturally, so they didn't understand how easy it was to repro. Since it didn't get fixed, I'm posting this. There are some other exploits/showstopper bugs that require switching characters during conversations. I'll make separate posts about those.
