I've been contacted by email by another professor at a different institution, who is assessing a student for graduate school entry, for an informal opinion about the student's suitability. The student hasn't asked me directly for a recommendation or similar.
Is it ethical for me to provide that informal opinion without informing the student? Does whether the opinion would be positive or negative make a difference?
I'm reasonably sure there are no legal issues involved, so I'm just interested in the ethical considerations.