There could be some plot behind all of this, but there is also a chance that everyone is making it more complicated than it really is. Hilary is an opportunist. She wouldn't be the first woman to have a male friend who treats her nice (friend zone) and another who she keeps for sex. I like to think that if Pip had actually been good in bed but still naive, she would still let him cover her at work and then screw him in the evenings.
However, Hilary is past the point of having any desire to train the newly post-virgin.
Pip's been shown to have questionable judgement, but he's never been shown as being stupid. How long things will go on before he finds out only Jack knows.