You know, I think Allison's ultimatum must have been strong because Gerard immediately ended that affair when she found out and he embarrassed himself by telling his co-workers and the school about it. I mean, he didn't love her he claimed, and had been wanting sex, and yet immediately turned around and ended it. Why? Did she threaten to expose him for having it and affect his so important image? Did she say there'd be divorce in no uncertain terms? Both? I'd say therefore if Allison had found out he was still having an affair that night such as via calls to his phone or if he really thought they'd run into each other the next day and she'd find out, that if he thought her threat was for real he'd have been worried. Once she found out she could tell people straight away, even before he knew she knew, and anything he might do to stop her actions after that would make him look more guilty immediately, so it'd have had to have been preventative.