[TOMT][Book] The MMC sleeps with the FMC's fake boyfriend

A while back (think years) I saw a reel of a book scene on Instagram where the mmc literally fucks the fmc's fake bf (She wanted to fool the MMC or something?) just to provoke and expose her i think or to force her to accept she's faking having a boyfriend who's secretly gay? If anyone know the name of this book please react ASAP!!!