To my knowledge when it comes to sex outside of a committed romantic relationship it's like this:
One-night stand = ワンナイトの相手 (wan-naito no aite) which means One Night Partner.
Hook-up/Booty Call/Fuck Buddy = セフレ (Sefure) a shortening of a shortening of セックスフレンド (sex friend), but really means a relationship where sex is the ONLY factor, and there is no social connection outside of that sex.
ALSO, there is a more intentionally vague term used sometimes, when in polite situations and desiring plausable deniability, which is 遊び相手 (asobi aite) which means Playmate. The term's meaning basically depends on who is using it: if it's a literal child or maybe a grandma, they could be talking about someone that they actually just do hobbies and play with. If it's a young adult talking about another young adult that they might be sexually attracted to, it's likely they are using it as a stealthy replacement in a polite setting for saying sefure because they don't want to be that overt and crude about it.
Friend with Benefits = Nothing really exists in Japanese, which is why they came up with their own term (The "Free Friends" thing, I haven't checked the raws to see what they really say). There really isn't a shorthand label for it, because while those relationships do exit, having a casual sexual relationship with a geniune friend is really socially ambiguous and not "okay" enough in wider society to be discussed openly. Hence the whole complexity of trying to break down the situation for someone else by describing it, and how they don't expect most people to be understanding.
Basically, the riskyness of the ambiguity of being in a real FWB situation, where it has a chance to turn really messy if someone catches feelings, the other person actually starts dating someone seriously, a new partner finds out their partner's bestie was also banging them, etc. etc. is not really kosher in Japan. So it's actually MORE okay to have a purely transactional but well-defined sexual relationship like a one-night stand, a regular hookup that you never hang out with otherwise, or hiring a straight-up prostitute, than it is to have a FWB.
So yeah, for English speakers, FWB is partially confusing because that's not what they are saying in the manga, and we have a specific definition of that term, even though Japan doesn't, and it's basically the meaning of the label they create for themselves. Most manga readers ARE familiar with the term Sefure, and that it's closer to a Booty call than a FWB, so IMO that would be a better term to use to translate セフレ into. Either Sefure, or Booty Call/Hookup/Fuck Buddy.
That nuance of the term FWB basically not existing in Japan, and it being a pretty big deal to have their own label they created to use, and that the co-worker actually not only comprehending their relationship but being okay with/not judgemental of it, imo gives a lot more impact to this chapter.
