This is a story I read in a gag manga.
It was part of the protagonist's daily routine to visit his favorite local coffee shop, enjoy a cup of coffee, and chat with the kind-hearted master and a young woman who worked there.
One day, the master had to step out briefly for an errand. Worried about leaving the young woman all alone, he asked the protagonist to watch the shop in his absence.
While he was keeping watch, a man appeared, and the young woman looked visibly flustered. As it turned out, the man and the young woman were actually lovers, but because of the master, they weren't able to openly date.
Assuming that the master and the young woman were father and daughter—and that the father was strongly opposed to their relationship—the protagonist stepped in. He told them that if they truly loved each other, they should be true to their feelings, no matter who stood in their way.
Deeply moved by his words, the young woman and the man resolved to elope together.
The protagonist told them not to worry, saying he would gently break the news to the master, who was her father, on their behalf.
At that, the young woman gave him a puzzled look and dropped a bombshell:
"The master isn't my father. He's my husband."
With that, she and the man walked out of the shop together.
Realizing the massive blunder he had just made, the protagonist frantically fled the cafe.
A few days later, he happened to catch a glimpse of the master, sitting slumped over in the pitch-black interior of the closed coffee shop.