Question

won't you warn her to be careful about what she said?这句话中won

Answer 1

从语法上讲,助动词 won't 修饰的是 warn 这个谓语动词,而非 what she said 这个已发生的事件,而 warn 还未发生（可能将来会发生）,因此用将来时.

补充回答：

在这段对话中,显然两处 won't you 都不对,都应该用 Didn't you,看两处的回答：of course I did / I told her...即可证明.