Advice needed from those with experience with parents of neurodivergent children/gentle parenting/how to handle bad behavior

I can't believe I'm posting but I'm so sad about my daughters situation with her 5 yr old son and his father. They don't see eye to eye on how to handle his behavior when it's clearly disrespectful.

So tonight at a family dinner I sat watching this scene unfold. My grandson has adhd and disregulation issues. He's in K and has already had behavior problems in school. So we all know this isn't good and how best to fix. Its dinner time and time to sit and eat. He has a lot of problems sitting still and eating his food. They allow him to run off his energy by playing...stopping to eat...then playing etc. Tonight he started to get too loud and wild. Parents are trying to redirect him without setting him off more. I suddenly see him jump up on the bench where he sits and putting his dirty foot on the table. Dad tells him to stop. He rubs his foot on the table as his response. Dad says more under his breath to the other adults at the table he needs something like a spank. Mom is triggered by this response as it goes against everything she has been trying to teach her husband and me as Mimi about descalating and grabs her son and holds him while lecturing her husband.

I'm watching this unfold and I wanted to cry over this. While I do my very best to support my daughter and break generational curses, I just can't help but feel like gentle parenting mixes with positive reinforcement and this plays into some of the behavioral problems.

I'm looking for parents who know what I'm talking about and have lived through this and share what worked for you.

Gentle parenting is based on the latest brain development research that wasn't out when I had children. That's why my daughter is on this path.

Old ways that did the yelling, time out, punishments, a smack do not work with him.

My daughter can get his behavior calmed and back on track but it only happens with her calming energy and support. I worry it's too rewarding for the bad behavior with all her attention.

Please don't respond unless you have lived this. Thank you.