If you ever (meet/met) her, tell her I said that.
which one is correct when tell someone the sentence above?
"If you ever meet her, tell her!" should be the correct simple version (for a future action). Because when you already met her, in the past, you won't be able to report her anything I said you would not have done yet if you are not supposed to have another date.
But we have: "if you ever met her, you know she's fine."
Otherwise, if you already met her, you know her, so you may be able to tell her what I said on your next meeting with her.
Your question has 2 senses.
If you ever (had already) met her, (you'b be able next time to) tell her I said that.
But if you have to meet before to say something to another, and not had already done, the rendez-vous is a condition. So we'd have to say : They would have met before...