I think it's good. The book is excellent and thoroughly gripping. Being a bit more..ahem...mature myself I can easily see how Yvonne got carried away.
She's had kids, she's worked for a Phd, she's risen to prominence in the scientific world, she's sustained a long term relationship with a faithless husband. She's done all the right things. And I guess she wants a break-to do something that's out of character, that's reckless and exciting. She may be only looking for self-affirmation that she still "has it", and X provides it, in spades. I think the "spook" thinking is simply part of the fantasy, as is not knowing X's name. I think it's rather poignant-a middle-aged woman who's achieved so much, has given so much, and maybe just wants something fun for herself for a change?
If twentysomethings get pissed, pee in gutters and indulge in casual sex no one bats an eyelid. Why should it be any different for a woman of a certain age?
Off-piste a bit, but wasn't "George" Mrs Tembe's love interest in Doctors? 
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Can't wait for next week.