I always thought "friend" refers to someone that you have a deeper relationship with than just hanging out sometimes and finding nice when talking to them. More like knowing them well, having them alongside you for important things in your life, trusting them to support you in difficult situations and also trusting them with more information than a simple acquaintance - discussing your emotions and what goes on inside you with them.
I need to have a reason for thinking that someone will never become my friend, but I usually wouldn't call people I work with, even people I've hung out with a few times, "friends" directly; even if I find them really nice and hope that our relationship will eventually develop to friendship when I get to know them better.
So somehow, the word "friend" has for me always been reserved for a longer, deeper kind of relationship. I hope that makes sense. But then, English isn't my first language, so that my be part of the reason.
I'm not sure if I would call Emily and Marten friends yet. Apart from the lake party, where there were many other people around, limiting their personal contact, they don't seem to have had any meetings outside work. Acquaintances who get along quite well and might become friends sometime, sure. Unlike Claire - she and Marten have had several quite private, close experiences together, so I'd definitely call them friends.
Anyway, probably not that important, but I think that is what the original post was aiming at.
Edit: Fixed typo