it may look like a bite but i dont think it is!
i'd say it may have started out as a small bite but has turned into a massive bacterial infection.
there are several reasons i dont think its a bite but the main one is the fact that this would have happend over days if not weeks and it would have been noticed
the scond reason is look at the fins ,,there practicaly untouched
the third reason is if it is a bite i belive the other fish would have sensed this perticular area as being a weakness,and what i mead by that is i had a zebra that had a massive tumour and all the other fish pecked at that one spot(so i sent him to a better place) so if your betta had a tumour of his own the that may have been the place it existed.
good luck anyway.