I would take a step back and answer a few questions first:
- What is the project's overall goal?
- Which strategy (the PR manager's or yours) aligns more closely with that goal
- Is this a battle you're willing to fight?
Ultimately you'd have to say that the PR manager is doing their job and you should trust them in that position, assuming you hired competently. However, they may be too removed from the project to understand your fundamental objectives. It might be a situation of reframing the strategy so that the PR manager can understand your position. However, if re-education is a failure and the PR manager is off base - ultimately fight your fight if you are the decision maker. That being said, for overall team moral you will begin erode the PR manager's trust and willingness to aid your campaign. If the issue persists, you may need to look at replacing your PR manager with someone that understands your objectives. But, make sure you're not just looking for a yes-man.