Stunt is real but only if you denied them with proper care (proper diet, water quality, right environment etc). But that gourami surely didn't looks like stunted to me.
He's not stunted, stunted fish look messed up. I doubt he's 10 years old.
Perhaps politely inform his owners that he needs a larger aquarium and offer to re-home him? He looks to be healthy, so I assume that they probably care for him.
Pearl gouramis could work instead, and will stay much smaller. Might be worth suggesting them.