His reputation took a huge hit when he sold out Diana. Called the "cad" in the media and so forth. Perhaps less money would have been better than money made by selling out Diana. Just saying. I think it's better to make money honestly than selling out a famous person for a tell all book. I doubt he wanted to marry Diana and I doubt Diana would have left Charles and the boys for him. She had the terrible example of her mother leaving the family for another man and her losing access to her own children.
I Dont think that Hewitt really saw her as a possible wife...
and he certianly wasn't going to "make money honestly" when he could do it by selling someone out. (and Frances didn't lose access to her children, she lost custody...)