I sure hope someone can help me. I've read the PBS "Help" and FAQs but still don't quite know the answer.

I received a book in bad condition (writing, underlining, water damage) and PMed the sender. She responded in a very polite manner explaining that she was sorry that she hadn't noticed the condition.

She's under the impression that PBS automatically returns my point, but I'm not sure if that's the way it works (believe it or not, in my two years and nearly 700 transactions on PBS, this problem has never arisen!).

In such cases, does the receiver automatically get the point refunded, or does the sender have to "gift" a point to the receiver via the "buddy" system?

Because she was so nice, I'll probably just tell her to forget it, but it would be good to know for future situations like this.

To get the credit she will have to buddy it over to you. Once you marked it recieved (even RWAP) the credit went to her account. From the HC:

The sender DOES get credit when the book is marked received with a problem.  The credit should be refunded to the requestor if the problem described is accurate.

Yes, she Buddys you, and then gives the credit.

She can look at her credit registry and she'll see there that she received credit for sending the book.

She got the credit (even if you marked it recv'd with a problem), so the only way you can get your credit back would be for her to buddy it to you.  That's actually pretty easy to do.