I agree, cops should be allowed to do this, however it can be an issue. For example, if a cop is aiding a fugitive escaping, other cops giving chase are put in a bad spot because they risk getting punished themselves if they hit the corrupt cop or even kill them by friendly-fire. Even worse, the corrupt cop may choose to die in a car explosion on purpose to screw over the police trying to stop them.
I could script it so that if a cop spends more than 1 minute in a vehicle with a wanted suspect then they are detected as working together.
That's a good idea. Here's what could happen:
If a cop gets spotted by other police officers riding in a vehicle with a suspect, he gets a warning.
After 1 minute, had he not left the car or ejected the criminal, he becomes tagged as a corrupt cop (maybe even change his name color from blue to another color).
If this happens, other officers can arrest him or take him down if he fights back, taking care of the "friendly-fire" issue. If he gets arrested X times for it, he could get demoted.