Is this ethical?
Researcher A asks Researcher B, "Where should I submit my paper?"
Researcher B says, "If you submit it to Journal X, I will almost certainly be the reviewer and I think highly of your paper."
All journals in the field have single-blind review policy. That is, the reviewer knows who the author is, but the author does not know who the reviewer is.
If it is unethical, is it only unethical if researcher B actually submits to Journal X and gets researcher A as a reviewer? Or does the ethical problem disappear if researcher B submits to a different journal, or submits to Journal X but gets a different reviewer?
From an academic standpoint- it is very dangerous. It is better not to do it, and it may backfire in a big unseen way.