You might be able to use that to help convince the judge of your innocence, but the law has no exceptions in it for "the cop asked me to." To be clear - if you stumble up on a scene and render first aid, or move traffic around (as a "normal" citizen would do) then you are all good. But once first responders are on scene, you no longer have that legal protection.
Think of it this way (and I exaggerate to make the point.) You pull up to an accident and somebody is gushing blood. You don't really know what to do, but try your best and the guy still dies. You are fine. You cannot be sued (well, I think maybe you can be sued, but it won't make it to a trial since you are protected under the Good Samaritan law.)
Now let's say you are driving past an accident and see a cop there. You walk up and tell him you've had training (neglecting to say it was just Red Cross first aid) and he says "go at it, the guy is gushing blood." You do your best, and he dies. You are now liable legally and can be sued.