a best friend

[Allthewayanime]

Is it acceptable to say 'She's a best friend' instead of ' the best..'?

[Beeesneees]

It would be acceptable if you meant she was one of several best friends.
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Cheers m' dears!

[Allifathima]

Beeesneees,
Does 'She's the best friend' mean
'She is the best friend out of all friends.' ?

[Beeesneees]

You would not use it to mean that as it stands.
If you added a phrase then it would work:
She's the best friend of all.
She's the best friend in the world.
However, neither of these mean 'she is the best friend out of all my friends', they mean 'she is the person who I think makes the best friend (to anyone).
_________________
Cheers m' dears!