Okay, you cannot and should not EVER tell someone their work is straight up garbage. It's downright disrespectful. I mean, you wouldn't want someone to say that to you, would you? (Speaking as someone who has experienced this first hand.)
What you can do is give them CONSTRUCTIVE criticism. Something that will help them GROW as a writer. Tearing them apart may help you vent your frustration but it's not going to help them IMPROVE. If anything, it's going to discourage them from continuing at all.
When you get that knee-jerk reaction, sit back and THINK about what SPECIFICALLY makes it bad. Is it the characters? Is it the plot? Is it the plot AND the characters? Is it the worldbuilding? Is it the grammar?
Once you've figured that out, IN SPECIFICS, you can figure out how they need to improve:
If the character is lacking in depth, then figure out what makes them feel like a cardboard cut-out. Is it that they feel like a YA female protagonist who has no motivation but to find the next love interest (Achievement unlocked: you found the love interest. Game over!)? Is it that they're a Mary Sue with no flaws? Is it that they have no clear motivation in general? Are the characters inconsistent? etc. etc.
If the plot is lacking, ruminate over the possibility that it could also be a character problem because almost all good stories are driven by good characters. If it's not the characters that are the problem, ask yourself: is the story cliche? Does it grip the reader? Does the story suffer from a lack of suspense? Does a story suffer from lack of CLEAR plot? etc. etc.
If the worldbuilding is lacking, you can ask yourself questions like: are the rules of this world unclear? Are the rules of magic unclear? Does the story only offer one type of lens to view this world through and the author doesn't show us the full scope? etc. etc.
If it's the grammar, you need to ask if the person is a native English speaker. Normally, when the author is not a native English speaker there are lots of grammar errors. If they're just lazy, that's WHOLE other issue.
If it's the drawing style, that's your problem.
Once you've figured out the SPECIFIC problems and questions you have with the story you can write a clear, concise review of the story that's not tearing into someone's story and just telling them "this is straight garbage."
Of course, there are those stories that are just not readable. You don't have to read those because the best rule to follow: if you have NOTHING good to say, don't say anything.