Potentially a lot if he intends to stay in that particular major and he loses. Architechture only has so many professors. In some of the more specialized majors it’s not uncommon to have the same professor again for a later class, not to mention professors do communicate with each other, and the potential for problems to spread.
Take also into consideration that architechture (and most other majors dealing with “art”) are always based on opinion. So the professor’s “opinion” is a valid bullet in handing out a grade, whether or not you agree with it. That’s just the way it is in architechture.
He could fight it, but the odds are not in his favor, unless something ethically blatant happened.