What happens if my friend opens my car door and the door gets pushed by the wind and hits a parked car next to my car in a car park?

YOU, as the driver, are responsible for the actions or failings of your passengers. Ethically and legally you are required to notify the damaged vehicle owner that your vehicle caused the damage and then either turn the claim over to your insurance company - pay for it yourself and have your friend reimburse you for their carelessness.