Re: Update to original post
Aren't we all over-thinking this just a wee bit? Isn't the obvious answer "exposure"? He's awake, she's oblivious to that fact, just going at it, doing something she swears to him she doesn't do. What's so hard about literal exposure? Whip off the sheets? She can't react fast enough to recover, as it were.
I'm guessing the phone thing is porn, not an EA or PA, because if she was so brazen she'd do that in bed, with her husband, I suspect there are many other wrong things in this marriage that would be worthy of splitting up. I mean really, if she believes her husband is asleep, why wouldn't she sneak off to the bathroom, or any other room in the house, where she could get off without having to smother herself with a pillow?