In a situation where you see someone acting badly toward a crew member you should not intervene directly into the situation. If the crew member is not handling the situation then find another crew member to help.
They are trained in how to handle guests. By putting yourself into a situation you will probably make it worse for you, the crew member and the irate guest.
The crew deals with unreasonable passengers everyday and they should receive combat pay for it.
Often the unreasonable passenger just needs to blow off some steam.