It's probably more that he's pleased at her progress than that her argument is particularly forceful, but he raises his hands in surrender. "As you like. Granted, it's much more fun in a drunken crowd, but..."

He clears his throat and launches in. He's not a trained singer, but far from tone-deaf, and his high baritone would certainly do for joining in a group. At first, he's mildly self-conscious, but his evident nostalgia for the song carries him through at least the first verse and chorus -- perhaps further, though that will depend on her reaction.