Well, the situation in the OP is not a FWB situation
but my f-buddy/ FWB rules are:
-Absolutely NO spending the night with each other. I don't spend the night at his place, he doesn't spend the night at mine. We have sex, then go our own separate ways no matter how late it is...this helps to make the time we spend together only about sex. Not too much cuddling or 'falling asleep in each other's arms'. In my opinion, spending the night (and smelling each other's morning breath) is a little too intimate for a f-buddy or FWB situation...it's only for people I'm in a committed relationship with.
-No calling or texting just to 'talk'. Whenever I call or text, of course I make small-talk, but I quickly get to the main point of my call/text...setting up a time to get the D.
-No showing up unannounced.
-No asking them to accompany you as your 'date' to any events or going with them to an event as their 'date'.
-The f-buddy/FWB can't be my only source of male attention or the only one on my roster. Meaning, while I have a f-buddy/FWB I have to also be seeing/talking to/dating at least one other man. This makes it a lot easier to stay detached and makes it so not all my attention is on the FWB situation because the relationship is not meant for that.
I know some people make a distinction between an 'f-buddy' and a 'FWB' and will say that my rules are more fitting for an f-buddy because I don't really make an effort to create a 'friendship'. But in my experience, it's pretty much impossible for me to both have sex and create/keep a 'friendship' without lines getting kinda blurred and conflicts happening because of some type of feelings developing. When I keep it only centered around sex and have other men I'm talking to, things are great.
I think it has to do with how you conduct yourself during the first couple of times you meet. Basically it seems like a good amount of the things I've read in dating blogs and books about 'how to act on a first date if you want a relationship' can be disregarded...not EVERYTHING, but a lot of it. If you want a FWB, you can talk about sex early on...like if he somehow steers the convo in a kind of sexual direction, just go with it. I basically just do what I want, act super flirty and find excuses to touch him when we're first spending time together.