Maybe the friend was newer to poker and was hesitant about the buy-in. Maybe the friend had lost an earlier big pot and was salty about it. Maybe your dad was a winning player and didn't want to clean house again. Generally when someone does something that appears irrational the problem is that the observer is not fully aware of the incentives that the 'irrational' player is operating under.

I've ran lots of home games where I will intentionally avoid isolating bad players when I have good hands, or I wont keep betting once I know I have it won, because I care way more about them having fun and coming back next time than taking their stack.