He expects to see her at the Christmas party, of course, but what he does not expect to see is the dress she chose to wear. The moment she takes her coat off he forgets everything and is completely deaf and blind to the crew surrounding him, focused only on the shape of her body. He doesn't know how long it takes for him to remind himself that he is married or to realize that there is a drink in his hand, but with a larger than usual swig he prepares himself for the thoughts that will inevitably invade his dreams and likely threaten what's left of his already fragile marriage.
Of course, she only wears the dress to impress Sherlock, who never has and never will see her in the way she does him, but it doesn't stop Greg from offering her a drink just for the chance to feel her skin against his fingertips.
"Molly," he whispers. "Do you want a drink?"
"Thank you," she says. "I wasn't expecting to see you. I thought you were going to be in Dorset for Christmas."
"First thing in the morning, me and the wife. We're back together. It's all sorted."
"No, she's sleeping with the P.E. teacher."
It takes a minute to let the news to sink in and another minute for Sherlock to further deduce Molly's attempt to impress.
The rest of the party doesn't go well at all until she finds him at the door on his way home. He apologizes for the consulting fuckwit's manner and expresses how proud he was of the way she stood up to him. It's the first thing that makes her smile.
"Greg?" she calls. "Do you want a drink?"