It would have been fun.

[keannu]

Given that there is no way of knowing whether it was fun for your friend (unless she already told you that she had fun), I find the use of "would have" quite odd.

My friend went to a concert yesterday. It was probably fun.
My friend went to a concert yesterday. I imagine it was fun.
My friend went to a concert yesterday. It's likely that it was fun.

I would only use "She would have had fun" if she hadn't gone.

It's a shame my friend couldn't come to the concert last night. It would have been fun (for her, had she been there).
If you went to the concert, you already know if it (the concert) was fun or not.

Then, what is this grammar stuff talking about?
http://www.englishclub.com/grammar/v...odal-would.htm

would: Presumption or expectation
* That would be Jo calling. I'll answer it.
* We saw a police helicopter overhead yesterday morning. | Really? They would have been looking for those bank robbers

[5jj]

You mean it's always a certainty for counterfactual hypothesis? The daughter actually didn't enjoy it, so Jane is just wishing the counterfactual fact?
No. Nobody has suggested that.

The reason I made such utterance is because of this grammar I found on the internet. It was to show "would have pp" in some cases can have the meaning of presumption. ...Isn't there any possibility that "would have pp'" can be a certainty without any conditional mood?
Yes, but the certainty is not as great as with 'will have + past particple', as a couple of us have explained.

5

[5jj]

Given that there is no way of knowing whether it was fun for your friend (unless she already told you that she had fun), I find the use of "would have" quite odd.

emsr2d2 merely said that she found the use of 'would have' in your particular example quite odd. I happen not to find it as odd as she apparently does, but agree that we would probably be more likely to hear one of the alternative versions she suggested.

We are dealing here with a construction that is fairly uncommon. Not everybody is going to see it in exactly the same way.