If they haven't asked me to read it, or tell them what I think, I would do nothing at all. Given any opportunity that qualifies as an "ask", I would be truthful.
I would not pretend to be any more expert than my friend, I'd just be honest about my own reactions:
I did not like the story, it seemed like the same story as XYZ. Or it seemed so predictable it wasn't interesting.
There were parts that made no sense to me, people were too stupid, or there were too convenient lucky breaks, or impossible guesses, or doing things for no good reason that turned out to be insanely lucky. It seemed unnatural or forced or too unrealistic.
The dialogue seemed forced (at specific points), and unrealistic, I don't think anybody would talk like that.
Whatever the flaws are, I am a reader, I have my own emotions and reactions to written material: My friend cannot argue that "you don't feel that" or "you don't think that".
If asked what I think of it, I would point out their problems that way, Here is my reaction to it.
If you are not a fellow writer, I would NOT get into suggesting writing groups, or specific fixes, or whatever.
These rules, btw, are not something I practice HERE, where people ask for specific solutions to problems (and I am a writer qualified to give advice). These rules are for a personal friend you do not want to lose, and do not want to start an argument with about the best way to write. It leaves you an out: If he thinks he writes wonderfully that's fine, his opinion does not have to change yours.