Thats pretty much completely wrong. You can earn companions from side quests and such, but not from battling a boss itself. I might be wrong, but from my knowledge of the game that's not how it works. Your friend is probably just guessing about it beacause he probably saw someone with one of those companions.
Some of the bosses you face can be recruited as companions if you have certain badges. Fin dorsal you unlock by earning the best chum badge (complete all fin dorsal related quests), chumba wumba you unlock by beating him 3 times. Eep opp ork ah-ah and gortez you unlock by beating each of them 25 times. The valvida brothers you get by completing the monkey's paw cave. All these you unlock in the crowns shop, and there are other badge-exclusive companions aside from these that are in the game as well, so there's that.