Any chance the snake is in the room while you were treating the tub. The fumes are toxic to them if it was.
Outside of that the only way it should harm you snake is if something prevented it from drying fully. I had heard of someone making the mistake of treating their water dish but the instructions clearly state you must remove it and not treat it with PAM.
Normally you treat the tub with the substrate in side of it. There is an applicaiton amount of 1 second per square foot. If you treat them seperate then the applicaiton amount may be higher. I dont see how this would be harmful if it was perfectly dry but I have never seen anything that says if too much dry PAM can harm the snake.
I did have the PAM people email me and say that if dry PAM is on substrate, and the snake ingests the substrate it will not harm the snake.
Their contention is if it is allowed to dry, it is safe. Even if it gets wet again it is safe. Once dry it is not suluable in water.