First, time for a sitdown with the in laws to determine whether this is possibly an elder fraud incident. Is Mr. DL really someone they know well or did they possibly buy the passes for even a small sum? It doesn't ring right that a 40-year employee would put his position on the line over a couple of buddy passes and it also rings false that a 40-year employee would tell anyone that they might be sitting in F when DL hasn't had international F for years. That's the kind of thing a regular DL traveler would know if he had stories to regale about!

Second, buddy passes have a value for those who have no set plans and don't mind waiting for days for a flight. Great for backpacking around Europe during college. Bad if you are 70 and have a cruise to catch. It's true that Mr. DL might be senior and those passes might give the in-laws a bit more priority than the ordinary standby, but not by much. They are still the bottom of the heap and all it takes is one cancellation and they may be stuck at ATL or at FRA waiting for 4-5 days to get over to VIE or back home. And that means sitting at the gate because standbys are called after the flight is boarded. Unlike revenue tickets, those passes won't be honored by any other carrier, so until DL has two seats, that is it.
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Third, if things go wrong, there is no help. No IDB, no EC 261/2004, and no travel insurance to cover. 4-5 days at an airport hotel + food can get expensive and at some point you pull out your credit card and buy one-way tickets.

Fourth, if you are the type who needs plans, these are not for you. If you have a train to catch, a cruise after that and if you need guidance from anyone, you won't get it on one of these passes.