I was once told by a government law enforcement official that a private citizens can beat a confession out of a perp and turn the evidence over to police and if they can go and get additional evidence they can make the arrest stick. Forced confessions are not allowed by government but no such restriction exists for private citizen. The private citizen would be liable for assault however. The case involved a family that had a child raped. They kidnapped the person that they knew did it and beat him until he confessed. The evidence gained from the confession showed he had done it and he was prosecuted.
Under such circumstances the issue of “duress” would be raised. I would be amazed if any honest judge would EVER knowingly allow evidence acquired from any forcible means regardless of who was involved to be allowed into evidence.