What do you think if I tell my friend that he might not be able to make a living off of his art
That's a little tricky. Why do you think you need to say something so critical?
I don't know, I just do. He can still try and  acknowledge that his artistic talent is limited.
It's not nice to crush someone else's dreams. You should be supporting your friend.
You are overreacting. I'm stating facts.
Do you have a background in art criticism? Many people told Van Gogh the same thing -- how do you think they'd feel were they alive today? I bet they'd feel ashamed at how they discouraged him.