There's a lot River could say to that. She could tell this man that Steve loves him, a lot (but he will say it doesn't matter). She could tell him that he is stronger than the words in his head. She could suggest that Wanda help him to with those words.

She could tell him that he is the best for Steve and probably will continue to be.

Instead she says: "Thank you," for his permission, leans up and kisses his cheek. The door comes open just then and she darts out before she can say, or promise, anything, clutching the box she came to the party with to her chest and mixing into the crowd.