I'd say do w/e you need to take care of ABC's. Ideally you'd want to leave the rider's helmet on and the head/spine alone until first responders show up with better equipment to help with it, but if it's in the way, try your best to remove the helmet without neck movement. Do what you have to do because it's better for the rider to stay alive. If you're in cell range, the dispatcher might advise you on the best course of action based on the rider's condition.
Another thing to consider is how many things might be going on with a rider in an accident. There might be so many things wrong that there really isn't one place to start and it might be a lost cause. Sometimes there's just already been too much much trauma. The sad part is if you're doing CPR, the outlook probably isn't good. Real life is a bitch
