This is awful and your parents are probably due some sort of compensation, but not under the Involuntary Denied Boarding policy which only applies to specific overbooking situations. Definitely try to get some miles or something out of this, but don't cite IDB compensation or you give the airline an easy way to deny you. I guess the lesson here is to always check your booking proactively and not depend on the airline to notify you.

One question though- did your parents book directly from Delta or from a travel agent (online or otherwise)? If the latter, it's technically the responsibility of the travel agent to inform the passengers of any changes.