So I just saw a question that asked for a recommendation. These questions are almost always closed because they require opinions. Is it a good idea to comment and tell the OP they may want to delete it before it gets downvoted as an opinion based question?
You can always explain to the OP that the post is not suitable and explain why (links to related questions/blog posts are always a bonus on these).
With luck, the OP will delete the post themselves, saving the community some time and effort.
If the post gets deleted, the reputation lost gets refunded, so that's not a real consideration here.
In short - it doesn't hurt, but you shouldn't stop with just a comment. Flagging the post as off-topic is the most correct thing to do.
I believe most important is to flag the question as off-topic, but also leaving a comment explaining why it is off-topic on Stack Overflow can be useful and kind, especially to help new users quickly understand how the community works.
Others might disagree about your comment, though.
I would not rely on the user in such cases. What if the user doesn't delete his question? What if he already has deleted a few questions and deleting that one more will get him banned?
The right thing to do is to flag it.
The kind thing to do is to leave an advisory comment.