Summary
In this oneshot, the guy calls their friend's father "uncle" (or Oji-san / Ajussi / Chú, depending on the language).
This is a common custom and a sign of respect, and it does not mean they are actually related (biologically) by blood. It's a way for a younger person to show deference to an older male family friend or acquaintance.
内緒で友達の父親と / Naisho de tomodachi no chichioya to