Maybe he didn't want to exert his rights and ducked instead so as to keep clear air? Is that considered a "Foul" by the port tack boat?
If the "right-of-way" boat changes course for strategic or tactical reasons, rather than as an "avoiding action" that does not constitute a foul.
For example, if S believes that the left side is favored (wind shift, better velocity, etc.), wants to go left, and bears away to prevent P from tacking on her lee-bow, then P does not break rule 10. If P hails "tack or cross", and S waves P across, then P is clean (even if S bears away).
I hope that helps,
Eric
