Thanks for the input, all. The opinion of poor treatment was from their removing her ticketed, checked in reservation from today's flight and shifting it to tomorrow's with no communication. I am not aware of any situation in which that's acceptable. The cancelled BRU-ATL flight mentioned, and trying to ditch lower yield passengers, certainly makes sense. After lengthy arguing and a couple of hours in line (the phone operators were not interested) they put her back on her ticketed flight with a 6.5 hour delay.
The inbound flight was delayed, but only by 2 hours. Delta has somehow managed to find an extra 4.5 hours of delay in there while on the ground in BRU. Rather than taking the airline's word for the delay reason, given that they have no incentive to tell the truth, is there any way to definitively find out the reason for the extra delay?