kids hooked up charger backwards

Well I came home from work and this kid put a new battery in his 85 d100 318/2wd and had it hooked up backwards, with a charger on it also. Now the alternator is not charging. I have a tester that will test at the battery for incoming current, am getting nothing. Did the old pull the pos. cable off with it running engine stalled. I put an old alternator I had on the shelf on still nothing. This alternator could be bad I honestly don't know. Has anybody done this before and what else could have gotten cooked? Voltage reg possibly. It runs perfect just no charge. Thanks for any help, If I can limit it to as few trips to the parts store as needed it would be great.