doesn't really matter, bottom line is it's hazardous even having it inside the cabin. now it sounds like he has an electrolyte spill to take care of but will deteriorate what it came in contact with, not including potent vapors inside the vehicle
this is one of the reasons you shouldn't have a wet cell style battery in the cabin of the vehicle. the other being the fact that they vent hydrogen gas.
sealed agm is the way to go and always secure batteries