How this “unethical”? Crappy situation? Yes but your post implies Delta maliciously pulled the plane to screw over your friend. But that’s pretty unlikely. Delta has no idea your friend was flying to see a dying relative. Unless it’s a charter, Delta probably has little to no insight on why any passengers are traveling specifically. And far all you know, someone on the MKE flight was able to make it to see a dying relative in MKE because of the swap. Here Delta made an operational decision, not an unethical one. For all you know Delta was carrying critical cargo to MKE or needed to get the jet to MKE to bring critical cargo back to MSP. What would have been less unethical? Putting passengers on a plane that had MX issues? This is not to say it doesn’t suck for your friend or that your friend can’t feel disappointed in the MX cancellation and swap or disappointed and frustrated at DL in general but to go as far as calling the swap “unethical” on DL’s part is way out there.