Yes, there are radar guns that law enforcement can use that allow them to determine the speed of a car coming in the opposite direction even if the police car is also moving.

Most moving radars display patrol car speed. Many moving radars measure on-coming targets only, some measure on-coming and/or receding traffic. Some moving radars can measure same-lane traffic in front of and/or behind the patrol car.

With other types of radars in moving-mode (opposite direction), the radar only shows the speed of on-coming traffic. With either type of radar unit the officer could indeed determine your friend's speed. State laws differ however in general an officer is not required to prove that a person was going that speed to the driver normally though the traffic ticket will have a notation of how the speed was determined (radar, laser, etc).

If your friend wants to fight the ticket then in court the officer would go into detail about how he or she concluded that your friend was going 70 mph in the 50 mph zone. A lawyer dealing with these sorts of speeding citations could give your friend more information on how radars work and if he or she would likely be able to fight the ticket or not.

Your friend will need to check with his or her insurance agent to find out if this speeding ticket will affect their rates. For a low cost car insurance rate click here.