There is obviously a lot more factors than a "guy" wants me to paint his house in exchange for this pile of wood. Now, the guy is your father's best friend. That changes a lot of things. Heck, I would probably paint the house of my father's best friend, especially if we are only talking 600 sf, for free if he bought the paint and supplies. Of course, I am pretty close with my dad and his closest friends. That isn't the case for everybody.
Being that you are painting the house of somebody that is your dad's best friend, there is that much wood in the pile, you can make money selling the wood, and you seem adamant about doing it (i.e., shooting down everybody that says do not do it because they make new chains every second of the day(lol)), I say just go ahead and do it. That one day of painting will seem like nothing if you are processing 30+ cords afterward. Buddy and I spent about 5 half days of work to process 10 cords.
End of the day, you know more about the friendship, etc. than we do, so there is a lot more at play other than "does this make financial sense".