How can the reader interpret Amanda's statement about a person's private life?

It can easily be interpreted as a warning to Victor that he does not yet know about her passionate tendencies. She is reminiscing about her marriage to Elyot (at Victor's bequest) and Victor is expressing astonishment at how she said the two of them had behaved toward each other. In fact, Victor later states that he wishes he knew her better, showing that Amanda could, unconsciously, be warning Victor about herself.