Then my fictive friend should write his own story, not claim to want to collaborate by trashing my stuff.
The largely-unspoken golden rule about adding a chapter to someone else's story is that nothing you do should immediately contradict or reverse something that happened in the story to that point. It's one thing to go down a different path, it's something else again to basically hit the undo button. At least, that was what I tried to do in LLNO when somebody else added a chapter to the story. Respect the direction they were going, even if it wasn't my particular kink or how I would have taken it.
I like to do my own stories but if I find something that's really hot and satisfies one of my dirtier kinks I do like to add my little bit to that sexy story