"her heart revelled in angry pleasure, in pleased contempt, that the man who at twenty-three had seemed to understand somewhat of the value of an Anne Elliot, should, eight years afterwards, be charmed by a Louisa Musgrove."

I have a different interpretation. Chap 4 says that Lady R wants Anne to marry a man of "talents and independence". FW has now proven that he is a man of talents by his success in his career, and he has made his fortune. lady R has seen the lack of adequate suitors for Anne over the last several years. Perhaps she had subconsciously been thinking tha FW and Anne might renew their engagement, and this time it would be acceptable. She was disappointed when he had paid attentions to Louisa instead, she felt contempt and anger for him, and consoled herself by thinking that she had been right in her early dislike of him, he does not really value Anne and could be easily charmed by an inferior girl -- hence the "angry pleasure" and "pleased contempt".