Who knows what happened, it doesn’t sound fair, but I don’t think she should have tweeted the woman’s picture. It’s the Delta personnel who allowed it to happen.
Around 1980, I was travelling with my 2yr 9mo old son from Dallas to SF on American. We had assigned seats. Got to the airport and checked in and somehow someone else had our seats. There were no other two seats together. They told me to board and the flight attendants would work things out. They didn't. The FAs made a half-hearted attempt to ask people to voluntarily move and no one would. So I sat in an aisle seat, and my two year old sat in an aisle seat on the opposite side about 3 rows back.
About 10 years later, I had a similar situation on TWA. The check-in rep moved us up into business class so we could sit together.
I've moved voluntarily. In a long line to check in, we got talking to a young black couple who had been on their honeymoon and were returning home. When we boarded the plane, the man was sitting next to us and his new wife was about 8 rows up. I asked my husband if he'd mind if I traded seats so the newlyweds could sit together to finish the last leg of their honeymoon. I traded seats and, in the no good deed shall go unpunished category, I ended up sitting next to a couple who were both extremely overweight and who were constantly getting up to use the restroom.