I have to believe that Captain Wentworth knew full well the possible consequences of his action but chose his duty to 'heart/friendship' over duty to others. This is the kind of person he is, which I think, is what makes it impossible for him to understand Anne's decision thus the bitterness, anger and resentment after so many years. He chose responsiblity to a friend over responsibility to his job. Yes, as you said 'it must have been a bittersweet joy, for she (Anne) once had that kind of friendship with him, and now does not.' I have no trouble believing he would have gone to the ends of the earth for her---in fact given his life for her (a la Tale of Two Cities). Not only that, if Anne had been his fiancee, she would have been his first priority. She would have been his 'heart'. I don't believe he would have betrayed her by taking such a risk if this had been so. But there was "no Anne". I'm not necessarily saying it was the right choice but it was his choice to take the risk and live with whatever the consequences might have been, a direct contrast to Anne's choice.