one I hain't NAER heard of....

The battery is a goner, and needs to be replaced. But before you install it, I would ask a few questions.

The first one is what is the last thing he did to the charging system and when ?

The most likely way for a battery to reverse polarity is to have someone cross-connect a charger (without polarity protection) to it when it is in a low charge condition. If he had a dead battery and charged it, that is probably your answer.

If he did anything else to the charging system recently, he may have reversed connections somewhere, which could also cause the reverse polarity of the entire charging system.

I have also heard of a few cases in certain rectifying schemes where a part (diode?) burns out and reverses the polarity.

So, it would probably be a good idea to check the polarity of the charging system immediately upon initial start-up.

The starter failure does not surprise me, particularly if it is an auto start generator. Once the start circuit called for a start, the starter motor would run backwards with the reversed polarity and the bendix would never engage. The starter motor would just keep running backwards until it fried.