Л-45

ЛЕЖАЧЕГО HE БЬЮТ (saying) mercy is or must be shown toward a person who is in trouble or who is suffering-he should not be caused further trouble: - (you) don't kick (hit) a man when he's down.

Конечно, она (Ольга Васильевна) выставила свою досаду совершенно напрасно... Потому что - на него свалилась гора. Лежачего не бьют (Трифонов 3). Of course she (Olga Vasilievna) shouldn't have let her irritation show...because he really was in trouble and you don't hit a man when he's down (3a).