Perhaps we say that we will try to build a friendly relationship without reference to their sin first, and then once we have that we will use the influence we have gained to begin to confront their sin.
What you have in the meantime, then, is a relationship that instead of being based on speaking the truth is based on hiding the truth.
So, when is the right time to stop hiding the truth and start speaking the truth? If it’s a Christian brother, he has already pledged allegiance to the King, so why would you wait? If it’s a non-Christian, his soul is in danger of eternal torment and yet you delay?