I think he stays on the p4p no matter what. How he loses would greatly affect where he lands on the p4p ladder.

Big one punch knockout... he has to stay up there, maybe landing Judah a spot on the bottom half. If he is thoroughly outclassed throughout the fight and then finished in the late rounds, Mayweather has to pay the price and move on down the ladder. And same goes if he loses by a lopsided unanimous decision in which he took a swift ass kicking from start to finish.

Personally... I think Judah winning would be horrible for boxing. We'd have a man with an invincible aura (Mayweather) getting a big L on his record and losing that aura. And we'd have a tainted product in Judah getting a big win, but still he would be damaged goods. It would bring Judah up a bit, Mayweather down a lot... and we'd have a scenario of meeting somewhere in the middle... and the middle is boring shat.