To your question, you may be able to damage a diode on the alternator, but probably not the alternator itself.

You have to wonder about some folks. Lets see...he has a charging system problem. That’s why his car is dead on the side of the road. You helped him. He now wants to blame you for the charging system problem. As if it was your fault he’s on the side of the road. Try, really, really, really sincerely, to talk some sense into him. If that fails, let him sue you. You can probably counter sue him for a frivolous suit if need be, and if you want. He’ll have to respond to the frivolous counter-suit before you have to respond to the suit. You’ll win on both cases. He’ll pay for your lost time from work, etc., in addition to his crappy charging system. Unless he's a real dick, he's not going to sue over a $500 $800 item. If he is a real dick you'll have to go through the motions no matter what, because that's what dicks do to other people.

There are also good Samaritan laws that might protect you. Check with your city/state about this...

BTW I am not a lawyer nor pretend to be. Just some common sense…

Best of luck, tread lightly!

…Tracy