Below is from "Friends(108)", an American drama.

Doesn't the underlined part have to be, "Whatever we pick, she will tell us it's the wrong one"?

The "picking" is not finished, so it's something in the future.

How can they say, "would've told", as if it happened in the past, when it is something that is expected to happen in the future?

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Ross : I have shown you everything we have. Unless you want your mother to spend eternity in a lemon yellow pant-suit, go with the burgundy.

Aunt Lillian: You know, whatever we pick, she would've told us it's the wrong one.

Mrs. Geller: You're right. We'll go with the burgundy.
Because the lady they are trying to choose an outfit for, has passed on. Everything she had to say, has already been said - and is therefore in the past.
Thanks a lot, Stormy...

I didn't know the lady had already passed on.

Is it to be understood as "Whatever we pick, (if she were alive and saw it), she would've told us it's the wrong one"?
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Yes, that is what it means.

The reason I can see she has passed on, is because of the sentence "spend eternity" - this means the afterlife, and that is why I can conclude she is deceased.
Thanks again, Stormy...

I understand your logic.... but...

Until now, I have learned that "would have PP" refers to past events.

But they are saying about future events, "whatever we pick....".

Can we make logic that "would have PP" can be used for future events?

In case the future events is of very low possibility?
It's not a future event.

They are speaking of a thing, which can not happen. But IF it did - she would have done XXX and YYY.

The reason this sentence is in the past, is because that is the only place this lady can participate in their conversation.

So - she would have done "so and so" but didn't, because she is now deceased.
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I see.... I see...

Thanks a lot, Stormy!!