We're a friend vs friends.

JungKimIn this context, can he say instead, "We're a friend"?

No.

Thanks.
Let me ask another question.

Now, one of the three friends says thereafter like this:
The man (to the girl): We're friends.
One of the three men (to the girl): Yeah, the three of us are a friend of his. // Yeah, the three of us are friends of his.

Are both correct?

JungKimAre both correct?

No; only the 2nd.

the three of us are friends of his. This sounds like the three are friends of a fourth person.

Clive

Clivethe three of us are friends of his. This sounds like the three are friends of a fourth person.Clive

That's exactly what I meant. The fourth person is of course the man who first talked to the girl. Please see the OP.

In this context, is it really not possible for one of the three friends to say "Yeah, we're a friend of his."?

JungKim is it really not possible for one of the three friends to say "Yeah, we're a friend of his."?

Yes, really.

No-one can say 'We're a friend of his'.

Here is what U.K. Defence Secretary Michael Fallon said:

Fallon says Britain will "stand up" to Putin's aggression.
"We are a friend of Ukraine, and will come to the help of a friend in need."

(Source)

Bait and switch.

JungKimWe are a friend of Ukraine

The United Kingdom is the 'we'. And the verb that agrees with 'we' is 'are'. This is British usage. Even the Queen of England calls herself 'we'.

Nevertheless it's still just one political entity, so it can only be one friend, i.e., 'a friend'.

CJ