Snape is over Lily. He's been coming to grips with losing his love for Lily ever since Harry gave Snape his (incorrect) explanation for Lily's treatment of him. The moment he kissed Rianne is the moment he finally decided to stop living and grieving for Lily and start living for himself.

The question is what it means for him to live for himself. Dumbledore's trust in Snape is based on his knowledge of Snape's undying love for Lily. If Dumbledore were to find out that that love no longer exists, he would (gently, perhaps) kick Snape out of the Fellowship. He would know that there was no longer any reason to trust him. And if Snape has gotten over Lily, he'll probably feel no real compulsion to help Harry in any particular way.

Quirrell, being a smarter Voldemort, knows what was driving Snape when he was working for Dumbledore, and he now knows that it's driving Snape no longer. That's why he had that little conversation with him in the woods; he knows that Snape is now a free agent who might once again be a blood purist loyal to Voldemort.

Voldemort would not likely welcome Snape back into his fold once Voldie reveals himself, but now that he knows Snape's loyalties are up for grabs, he won't hesitate to manipulate him and use him however he can until then.