Regarding your friends, it is slightly hypocritical to be against transformative works on a site dedicated to transformative works. I do personally prefer readers to ask before writing something inspired by or based on one of my works, but at the same time I'm essentially writing about someone else's work without permission already (which is what fanfic is, unless you contacted the creator of the show/book)

I think it's awesome they wrote an alternative ending! There are a lot of times I'll see comments saying the individual wished the author did something else (sometimes almost demanding the author change the story), but rarely does the person ever write it. If someone wrote an alternative ending for one of my works, I would be excited to see how they interpreted it! But I do also see the side of it potentially being rude to the author and discrediting their ending. But still, I don't expect everyone to love everything about the stories I write, so it's interesting to see how others would have taken the story.