KLM/DELTA DON't VALUE PASSENGERS

My sister booked on KLM to visit us in the USA...Flight confirmation code/number 3LLSXI. For her return flight Houston to Amsterdam she was allocated seat 36C, an isle seat, like she requested.After a nightmare experience in Amsterdam, her flight to Houston got cancelled and rebooked for the following day,we've decided on May 3 to call to upgrade her seat from Amsterdam to Cape Town.After more calls, we've received the e-mail the next day only, that I've requested.We were shocked to see that her seat 36C was changed to 36J, a middle seat:( after it was confirmed on May 3 by Delta.I've spoken to a guy named Dogg, from Delta, who refused to give me his surname.He had the can't care attitude and said she can sort out her seat at the airport. She has wheelchair assistance for her trip. She refuses to sit in the middle and just want her seat back that was allocated to her in Cape Town. I have e-mailed KLM and also sent an e-mail to their president, but still waiting for a reply. This experience is a total nightmare for my sister, LE Aburn being her 1st international flight. She will most definitely never fly KLM again and neither would any family.They don't value their passengers