I wonder if this could work: Think of someone else in your life who does this to you. (One-upper #1)

Then vaguely tell one-upper #2 about your struggle with one-upper #1. Then ask #2 how they would deal with #1.

Would this give #2 a chance to hear how this behavior can affect the listener, without putting them on the spot or embarrassing them? And maybe initiate some self-reflection?