As someone who witnessed the personality change in my mother when she married a man who was clearly wrong for her and her unhappiness for years as she stayed married for the sake of my half siblings I determined I would never do the same.
The atmospheres and passive aggressive behaviour, sulks, simmering silent anger made me a bag of nerves, very confused and I certainly suffered a lot from the stress of it all so I would walk the streets after school as I could not face going home [and got into trouble in many ways doing so].
So after trying so hard to sort the problems when my ExH continued to constantly bicker, criticise, moan, show disinterest in and bully his 2 little boys due to his resentment of being a parent and having bills to pay etc....I called it a day. Sons were just 7 and 4.
The boys had been fighting and arguing, it was escalating as he was on their case every day when he got in from work. I was not going to see them subjected to emotional abuse as I had been [long story], I was just about coping myself. When it was just the 3 of us we were fine and happy.
When he left the atmosphere lightened, the fighting between them decreased dramatically, we were now in a peaceful home and Ex saw them at weekends when he treated them with more care now the pressure was off and took them to their nan and gramp's house who they loved.
My 2 grew up fine, I had no problems with them as they grew up though both wished things had been different at times. We would talk about it, and as they got older they could see how disconnected their father was. One was annoyed that he claimed free school meals for a while but laughs about it now! Both are now happily married with 2 children, hardworking, well balanced men. My friend [with your relatives opinion] who hung on for years had a worst time as her breakup when her boys were 13 and 15 was dramatic, intense therapy was needed, one hated his mother for years and the other had substance issues and took a manual job instead of continuing studies [he was bright] just to get away from home.
Your relative is not right as every case is very individual, based on circumstances and personalities. I think it is better if the children are younger and if the parents are civil with the children's interests a priority.