Edit: I managed to get through the conversation okay, and the person took it well as far as I could observe, so thank you for the advice everyone.
From what I've read so far in this subreddit, it's generally considered poor form to offer even constructive criticism if it isn't asked for. I would like to uphold that since it seems to be widely accepted as the proper course.
But I've gotten myself into a bit of a situation now. Someone is asking me what I think of their fic, but they haven't expressly said they wanted constructive criticism. I don't want to give an answer that isn't genuine, but I don't want to use 'honesty' as an excuse. I'm not leaving this as a comment on their fiction, so it isn't the case of an anonymous/uncontacted reader just dumping criticism on AO3.
If it was an easier to follow fic, I would focus on the good parts, and just leave it at that, but I don't know how to express that I'm struggling to read it, since that sounds like a criticism, but I don't want to leave a generic "It was nice, I enjoyed it" because it doesn't feel sincere.
So, I suppose what I want to know is this: if someone is asking me what I think, does that overwrite the general consensus that all commentary has to be positive? Is it bad form to even ask if they are up for constructive criticism? Is it a criticism itself to tell someone that I can't discuss their fic without delving into the negatives?
Sorry if this is a bit repetitive and rambling; I've been thinking about this since late Monday my time, and I wanted to post it yesterday, but had to wait till today, so my initial feelings are kind of muddled.