Your friend is not required to prove her innocence. The Crown are required to prove her guilt. The onus is on the Crown to find the evidence to support their accusation that your friend was using a mobile phone whilst driving. Remember, innocent until proven guilty. Your friend should plead not guilty and sit back and allow the prosecution the opportunity to prove what can't be proved.
Of course, two police officers reporting that they saw your friend with the handset pressed to her hear whilst driving would be quite compelling. What did the police officers report?