An Old Fan Fic Problem

I was in my second year of college when I became good pals--and hoped to be best friends--with my GM. I arrived at his house early by several hours and I spent the time chatting with his siblings. Now aside from my GM, I was probably closest to his younger sister, mainly because we shared the same interests. Her name--an alias as well as the rest of the people in this story--was Jan and her best friend, Kim, was there as well. The three of us talked for quite some time before we broached a topic about a mutual friend, Patty.

Now Jan and Kim were good friends with Patty while to me, Patty was simply someone who mingled in the same social scene but never really someone I was ever close with. The problem started when Kim asked Jan if she read Patty's recent fan fic and how she was plagiarizing another fan fic author's work. I asked, whether it was me thinking the best in people or simply playing devil's advocate, if this was verified. Soon, we were all on the Internet and we compared Patty's work with the other fan fic author and there was indeed a passage or two where the two were perfectly identical. The next step was to check publishing dates (as far as checking online publishing dates go) and it seemed Patty's fan fic was the later work. Despite the evidence, I asked whether we should ask Patty herself about it.

Jan and Kim spoke a unified "No!" and to me it seemed that they were over-reacting. Nonetheless, Patty's actions disturbed them so I sought to put an end to this mystery the direct way. I got Jan's mobile phone, because I didn't have Patty's number, and asked whether she did indeed plagiarize someone else's work. I don't quite remember if I attached my name to that initial text message but there was no doubt that in the subsequent text messages, I identified myself as the sender.

Apparently, Patty took offense so much so that she never responded to Jan since then (and no, that is not exaggerated--a text message or two and Patty ceased talking to her "best friend", no explanations asked). I apologized to Patty and said that it was me who was asking the question, not Jan, and if there was anyone to blame it was me but she didn't respond to my text messages either. So in one fell swoop, Patty was cut off from my life and Jan's. (For the record, there were several incidents later in which Patty would be accused of plagiarizing the works of other fan fic authors.) I also apologized to Jan and it was only then that I realized why the two friends didn't want to confront Patty in the first place.

In retrospect, I shouldn't have butted in the first place. Then again, I never really expected Patty to be actually guilty of plagiarism and a simple denial that she had done so would have satisfied us instead of the silent treatment lasting several years.

Right now, I have two questions:

1) Because I'm not well-versed in the fan fic scene, is it ethical in the fan fic community to copy-and-paste another author's work (without permission)?

2) If you were in my position and you wanted to find out the answer to this mystery, would you have a) asked Patty directly, b) base your conclusions solely on the evidence provided, or c) discover some other way?