I'm frustrated with my friend. He keeps checking his ass out at the gym.
That sounds weird. Why do you think he is checking out his ass?
It is so obvious. He keeps glancing at the mirror checking his ass
It is understandable to be upset about your friend acting weird in public. People will think you are the same and think the same about you.
He is not a narcissist. He is just my friend who is checking his ass.
Even if it he is not a narcissist, you can still be upset if he is embarrassing you in public. You could be affected by him making a fool of himself while you are with him.