What do you do if you accidentally put petro in a diesel car?

It depends how much petrol was put in. If it was a "full tank" draining the tank and the system would be advisable. If it was just a small amount, the mixture can be cut by adding more diesel with some motor oil mixed into it It is a common practice in times of diesel shortages to use motor oil with a small amount of gasoline added to it in place of diesel. You will have to use your own judgment as to how much diesel and/or oil to add.