He should have been warned of the danger,so he was.
他应受到警告有危险,他的确收到了(警告有危险)
这里省略了So he was (warned of the danger)他的确收到了警告
如果用should的话,应该是是He should have been warned of the danger,so should he.
他本应该受到警告有危险,(另一个人)他也应该(受到警告有危险).
这里省略so should he (have been warned of the danger)