Some girls know the game very well and it is true the harder you have to work the more you value the girl.

The problem with this is that it creates a unconscious challenge. The guy fights to get her in the sack, he gives her extra attention in efforts to overcome the challenge. Then when a woman finally gives it up, the challenge is gone. ( his efforts deplete) Couple this with her passing her own psychological barrier and it can be a recipe for disaster.

If it's been too long. Ie: a couple of months. Their entire existence up until that point could of had the emphasis of the challenge.(sex) So once the woman gives in, her mind set is "ok this is the real deal" and tends to become ultra submissive, but because the challenge for the man is now complete, he will unconsciously tone down his efforts and completely throw the woman for a loop. ( she will become needy, and question his lack of effort that she has become used to a certain level of attention)

If there is not a good level of communication between the two prior and present, the lack of effort from the man can often be seen as lack of interest.

I have seen this happen way too often. The woman becomes so used to the attention, that she associates that with his desire for her alone, once it tones down, she becomes an emotional mess. Questioning everything, this in turn can push a man a way very quickly.

My Advice you your friend, If he really does like this girl and wants to have a relationship with her. He must keep the lines of communication very open and try to keep his vested interest in her at one constant level.