TBH my headcanon is that Khadgar has bedded just about every important lore figure, man or female and that he is known very well for this trait,
I mean, come on. He's the literal definition of a silverback charmer.
Depending on interpretation of various quest and npc text throughout WoD it's not even headcanon that Silver Fox favours and grooms you with attention, jokes, food, wine (in nagrand) and gifts (ultimately a ring). It's straight up stated.
My headcanon is that Cordana was already a recepient of these attentions, henche her unprofessional overprotectiveness. Seeing Khadgar's attention shift to a new squeeze, she despairs but keeps a strong face.
Seeing Khadgar's obssession with freeing Garona at any cost (new, pliable toy? While still courting Player?!) Cordana succumbs to her darker urges, using the orb to join the demons in a fit of despair. This love triangle is not elaborated upon beyond Khadgar's sadness to see her go.
This explains the lack of stated motive for her betrayal and seething hatred of the Player in Legion (having stolen her Fox) to the point where she sticks around the warden prison just to fight and kill you.
Khadgar continues his ways with the obvious Modera connection while you're combing the isles for treasure (so far from him) and later low key dumps you by having you fetch coins to gift his... someone! You're so dependable, after all. Player is too busy stopping the Legion to confront him about this but like Cordana, keeps a strong face.
This is why Player abandons Khadgar's dalaran and order hall neutrality in the end, wising up to his ways and taking out their frustrations on the other faction. In retrospect, there were three "savage hallows". Three rings! You were played all along...