Job One: get the friend out of this equation. This is not the friend's business. Don't ask for advice, don't ask for confirmation, and for God's sake don't behave like a second grader and ask the friend to convey messages in either direction.
Job Two. Ask your potential paramour - you know, like when you're together, because you're FWBs - what's up with us? Are we a couple, or not? Are we dating, or not?
Nothing about feelings; this is about definitions only.
Job Three. Vow to yourself to never, ever get involved in FWB again, because it's exceedingly rare that both persons are on the same page. FWB is generally just a way for people to do the horizontal tango while rushing headlong into really hurting one or both parties, and without the annoying baggage of dates, meeting friends or family, spending money, putting on makeup, shaving, or otherwise making an effort or treating it as anything other than Rent-a-Set-of-Genitals-That-Aren't-Your-Own. FWB, inevitably, is usually one person la di da enjoying FWB while the other one is moping around and wishing and hoping that it would somehow magically turn into more. If I was only more available. If I could only play hard to get on occasion. If I were only more mature. If I were only younger. If I could bring myself to sexually experiment. If I could only put the brakes on the screwing and insist on a real relationship
If if if.
It's magical thinking, is what it is.
Stay out of FWBs. They mess with most people's heads something fierce.