ANSWER:

If this is going to open his eyes and make him stop, this is fine. However, if this is not going to change anything in his behavior, this is not good for his dawah and would only result in him getting further from Islam.

Try to be diplomatic and wise in the way you deal with him while anticipating the consequences of your actions on him. 

My adult blood brother is having an inappropriate relationship with a girl. May I, without breaking ties, turn away from him ( not speak with him unless necessary ) in protest after I advise him? He is not a Muslim but believes in Allah.