if your friend asked you to read their work for corrections,... then do so,.... they have entrusted you, their friend to a huge task,... don’t let them down. they’ve trusted YOU,.. live up to that trust.
help them fix spelling and grammatical errors, and rather than pointing out story line problems, instead you might ask questions,.. questions that might come to the mind of the average reader,.. and the answer to those questions will fix the story line issues,
don’t let your friend self publish something that is less than they should,.. don’t let your friend spend hard earned $$ on an unfinished product,.... however,..... if the story in and of itself is awful,... help with spelling and grammar and time line problems,... but keep the OPINION to yourself,.. its your job to fix spelling, grammar, and time line,... don’t explore characters, or ask existential questions.
hope i helped