I don't really see this as all that crazy. How would people prefer this situation be handled? For them to assume that they did have sex and DF her anyway? Its pretty suspiciuos but there's a lot of reasons you can be alone with someone for an hour and not be having sex. So yes, if you're careful you can get away with an affair because there are no witnesses. Thats the lying, scumbag partner's fault... no one else's.
As for the all night rule, I suppose its exact definition is a little arbitrary but honestly, if two people go home together, go to sleep, and wake up the next morning and leave, what are the chances they didn't have sex? I mean, what other reason for that occurring would there be? Its not like going behind closed doors for a while where you COULD be doing anything... kissing, looking at baby pictures, just talking, getting a massage. There's only one reason you're going home with someone all night.
I don't think this is that crazy a letter really..