Can a police officer open your car door and pull you out of it without giving cause?

Civil rights laws are in place to protect against those who act in violation of constitutional protections under color of law, such as police officers. From what you wrote it is not possible to know for sure if your friend has a case. I would say you do not have a civil rights case, because being a witness to your friend being pulled out of the car violated no constitutional right that you are entitled to.

If your friend wants to check about his legal rights, have your friend check with a lawyer in your locale to discuss more of the details.

Good luck to you.

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Was it that simple? Why were you stopped? Did the friend provoke the officer? Was he trash taking the cop? All of these things may explained what happened but based on what you have said it's impossible to know if there's a civil rights claim. There's a better chance there's an excessive force claim against the officer.