This is what happens when one’s social circle dates primarily within itself:
D used to date S.
D also used to date B.
S is in a serious, but open relationship with W.
W is courting P.
P is FWB with B.
It’s not so much a dating circle as it is a figure 8 - or something. Fortunately we’re all adults (youngest is 27, oldest is 39) so there’s less drama than you’d think.