Zài péngyou zuì kùnnán de shíhou luòjǐngxiàshí, shì bù dàodé de xíngwéi.
Kicking a friend when they're at their lowest point is immoral behavior.
Tips
memory
Literally: 落 (fall) + 井 (well) + 下 (down) + 石 (stone). Imagine someone tumbling into a well, and bystanders throw stones in on top of them. A vivid image for piling on someone already suffering.