That it didn't work out. The rest is none of their business.
Yes. Otherwise they'll get deeper and deeper into it. At lease they have a fighting chance if you tell them.
It can be challenging to determine if someone is pretending to be heterosexual, as individuals may have various reasons for presenting themselves in a certain way. However, some signs that a gay narcissist might be pretending to be heterosexual could include inconsistencies in their behavior or statements, reluctance to discuss their personal life, and a lack of interest in forming intimate relationships with individuals of the opposite sex. It is important to approach these situations with empathy and understanding, as individuals may be struggling with their identity and facing societal pressures.
Tell him to go and leave you alone.
If he or she dates people of the same sex.
A - How do you know they are narcissist? B - Why do you assume that the friend doesn't already know? C - Why do you think they might want to know? D - Is it any of your concern? If you can satisfactorily (and honestly) answer those questions then yes, tell the friend, otherwise leave the matter lie.
Sometimes not at first because a narcissist likes drama and likes to be around special people in order to feel special vicariously. But if and when she becomes too much for him to deal with he will leave. A narcissist really needs submissive women. Women they can dominate and control and get "Narcissistic supply" from....at heart they are insecure cowards!
Be careful if you do, the narcissist can be a monster to anyone who hurts their pride.
On the contrary, most families leave the narcissist.
I think this sums it up nicely ... http://www.ultimate-self.com/controlling-people/
Leave, then empty the bank.
It's not best to tell him after a while so be straight and tell him
leave and/or abuse you and blame you.