At what point did Madame de Merteuil 'turn' against Valmont?

We see the ageing lothario and courtesan as former lovers agreeing to co-operate upon a plan of revenge against their own former lovers. Clearly this is a malicious scheme to humiliate and degrade people.

But somewhere between that spiteful beginning and the embittered ending, de Merteuil declares "war" upon her ally/lover/rival?

I think it was that she was wounded and enraged by Valmont's pet project, de Tourvel?

He dashes back excitedly to de Merteuil's chamber and- instead of revealing how it was nothing and how he dumped his pet- he stares out of the window and extols the virtues of his 'love' with Tourvel, clearly carried away and failing to notice the obvious heartbreak of Mertuil, who sits rigidly and fades into a griefstricken trance, given her mortified facial expression. Something a woman 'in control of her emotions' would never allow?

"I've never known anything quite like it" he clumsily and wistfully says about his 'love', forgetting how such insults will infuriate a vengeful woman.

I think it was here that de Merteuil hatches her plan to set Valmont up and ruin both him and his 'love', as displayed in some of the later scenes, especially to get Valmont to break with Tourvel - which he expresses his deep regret for as he dies?

Opinions?