The phrase “critical friend” is sometimes used to describe the role of a trusted adviser and, in particular, one who is prepared to tell you something you might not really want to hear—but do need to hear.
The role is reasonably widely understood, but that doesn’t mean it’s a commodity, or even really a service to be bought. It does depend on there being a real and genuine connection between the people involved. They need to care.
To be a critical friend, you need to start by being a friend.