I think if I were responding to the question I would want to refer to how Leila feels isolated at the beginning of ball through her lack of experience and then feels isolated again and alienated after her dance with the old man and the unwelcome reminder he gives her of the ephemeral nature of youth.
At the very beginning we see that Leila is presented as isolated as she goes to the ball. She is "in her own little corner of " the cab, and her isolation in terms of experience is clear when we discover that this is her first ball due to the way that she lived 15 miles from her nearest neighbour. She likewise is presented as isolated because she is an only child and stares enviously at the Sheridans:
Oh, how marvellous to have a brother! In her excitement Leila felt that if there had been time, if it hadn't been impossible, she couldn't have helped crying becaused she was an only child and no brother had ever said "Twig?" to her; no sister would every say, as Meg said to Jose that moment, "I've never known your hair go up more successfully than it has to-night!"
Seeing the sibling love of the Sheridan's makes Leila feel even more isolated and alienated than she felt before.
The second moment you would have to talk about comes after the old man who dances with Leila reminds her that her youth and grace is just temporary, and that before long she will be a chaperone on stage. His words make her want to stop dancing:
But Leila didn't want to dance any more. She wanted to be home, or sitting on the veranda listening to those baby owls. When she looked through the dark windows at the stars they and long beams like wings...
The reminder of the brevity of her youth produces a desire within Leila for solitude and makes her embrace isolation, as the little girl inside of her wants to throw "her pinafore over her head" and cry.