Frontier Airlines is adamant they were NOT in the wrong when they booted UFC legend Khabib Nurmagomedov off a flight this weekend ... claiming the 29-0 fighter ignored a flgith attendant's instructions before takeoff.
But, TMZ Sports just spoke to someone close to Khabib, who says that's simply false.
First, Frontier's side of things. The airline says Khabib -- who was seated in an exit row -- failed to indicate he was willing and able to assist in the event of an emergency.