Which is some revisionist history that ignores what actually happened, as Rick knows that Daryl showed up with the medicine they needed not long after Carol did what she did. He may have meant that he would have made the same decision as she did at the time, but saying that she was right to do it just ignores what actually happened.
They never knew what specifically caused the virus, or what it was; it was all guessing. It would have been mistaken to throw the term "flu" around, when flu as they knew it never caused such extreme reactions such as eyes to blacken and bleed. It was pure luck that the very late drugs Daryl's crew retrieved worked at all, but one thing was certain: while Daryl's crew was away, Hershel was faced with more deaths from the disease, and yet another walker outbreak, since no true contingency plan for the infected was ever put in place, so the only way to stop it was Carol's way.