Is "best" still a superlative in "best friend", as in can you have more than one "best friend"?

I was speaking to a 15-year-old native English speaker (in Australia), who referred to someone as her "best friend". Later, she revealed that this wasn't her only best friend. She had four best friends.

She couldn't understand why that was confusing to me (and I didn't want to get into a discussion about "best" being a superlative for "good").

Is this just one girl who is using the term unconventionally, or has the term "best friend" been softened to only mean "very good friend"? (Or even more extreme, has "best" been softened to only mean "good"?)

There are several things happening here, I think.

First of all, a superlative does not always have to literally refer to a singularity. Superlatives are commonly used as amplified comparatives. This can, as @Oddthinking remarks, be seen as hyperbolic use of the superlative:

We had the best time last weekend!

That doesn't mean we necessarily had a better time last weekend than we had at any other point in time. Likewise, a best friend is not necessarily a better friend than all others.

Secondly, as, others have said, best friend can be short for one of my best friends. Superlatives can be used to describe a group of items:

The three best books I have read are ...

And finally, yes, some phrases suffer semantic deflation. Even assuming that best friend can refer to one of several people, the expression best friend forever (BFF) has been in use for a while in on-line media. Interestingly, people report that they have "a new BFF", which seems contradictory to the strict semantic interpretation of the expression. It seems people use the expression rather as "a person who they see as quite a good friend at this moment", rather than "the person who will for all eternity be my one best friend". Ah, well, language is the darnedest thing!

Best friend is defined by WordNet as:

the one friend who is closest to you

Longman defines "best friend" like this:

best friend: the friend that you know and like better than anyone else

Strictly speaking, these definitions imply you can only have one true best friend.

That said, I do agree with others that it's possible to talk about several "best friends" in normal colloquial speech. Evidently, people use the expression more loosely compared to dictionary definitions.

I don't see what the quandary is here.

No matter what the criteria, there's always the possibility that two or more people will score exactly the same on them. Two people may throw the same (longest) distance at a championship, which entails two winners and two best throwers for the time being.

In a more abstract way, the ordering which enables us to speak of best at all, usually admits that there can be more than one element at the top.

If the criteria are many, fuzzy, or difficult to pin down, then the qualification of being "best", I'm afraid, follows suit, making it even easier to have a cloud of the best elements.

And if there are no criteria, and one can "feel" what makes someone one's best friend, then, perhaps, one can sometimes "feel" that one has multiple best friends, too.

Teenagers will say anything.

Actually, there's nothing wrong with saying "one of my best friends." And if you'll recall the Musketeers, there were four of them.

But generally, yes, when a girl says "She's my best friend," she should be implying that the person she's talking about has a unique place in her life, above her other good friends, pretty good friends, so-so friends, and just friends..