Well, yeah. you do seem a little naive. A one-night stand is with a stranger.I really don't think it is unreasonable for him to expect that she wouldn't be out having ONSs while dating him, or dating others without saying something. Maybe I'm just naïve.
The husband was more of a stranger to her at the time this happened and you continue to use the term 'dating' as if they were physically and emotionally connected in some way because they WENT. ON. A. DATE.