Battery issue

[schwalbach]

So been camping with my buddy since spring, noticed smoke coming from his battery box. pulled cover and disconnected ground lead, battery was so hot it felt like it could melt, steam coming out of the battery. After looking things over we found the battery was put in backwards. Went and got a volt meter and the battery had 6 volts negative voltage. and yes I know how to read a voltmeter, black to black and red to red. Checked inventor output and found it was charging at 13.4 volts. But he had the red hot positive lead connected to the battery negative and ground to positive.

Only thing I can think is battery was stone dead when he put it in, over time hooked backwards it desulphated and charged reverse polarity, I have heard of this but never actually seen it happen live. So question is-how did he get by for the last several months? Also rechecked battery after sitting awhile and it cooled down but still had negative 6.4 volts?

Anyone ever actually seen this?