She wasn't entirely expecting him to know it, so she's impressed. She lets him do the words and hums a harmony along until he misses the line, then fills in for him.

"...would never come, I'd see delight in the shade of the morning sun."

She finishes out the chorus and then lets it peter out.

"Nice, Dean," she says. "Little rusty but I dig it." She does actually smile, just a little.