Hmm. When in doubt, gender swap!
So a cute?, little bit wacky, and not very bright Magician girl, likes a very shy, tsundere fighter class knight. And even though, feelings started to grow on the knight's side too, this good looking dark elf jock, begins to heavily flirt with "her". And "he" isn't stopping, even though he knows about the relationship between his friend "the knight" and the magician girl, because "she" is too cute and "he" just wants her. Yepp. NTR in progress.
But said Knight is taking way too long to get active. He has no write to complain, if the girl decides to go with the other guy, even if it's just for a night.