Can I also say "He did what one should have done."?
Hi, Subhajit: That one doesn't work so well, unfortunately. I think what you mean to convey by that sentence is what the following sentence actually expresses: "He did what any decent person would have done." And that sentence can have either "would have done" or "would do." With "would have done," the sense is "would have done in his shoes." With "would do" ("He did what any decent person would do"), the sense is "would do in such circumstances."