Some of these comments are so ignorant and judgy, the typical "I can't imagine anyone not being exactly the same as me and living the exact same lifestyle as me" Mumsnet mentality.
My parents divorced when I was 12, and died when I was about 30, and they were very good friends and got on very well for the rest of their lives after the divorce and would go to a coffee shop to exchange presents on each others' birthdays every year. Definitely neither of them had feelings for each other, neither of them had any interest in getting back together, it certainly wasn't some "selfish woman daring to want to be with her own child" thing since I was an adult (and often not even there), and I didn't find it confusing in the slightest, I thought it was nice that they were able to be friends and I never regarded it as playing happy families or confusing.
Obviously if there's been any form of abuse or if you hate each other then of course not, and I wouldn't expect anyone to hang out with someone who cheated on them either. Most marriages so break down because someone has behaved badly or because there's bitterness, so most couples aren't going to still be friends and that's fine. No one should be expected or pressured to stay friends with an ex.
But for people like my parents, who got on extremely well and liked each other, no abuse or cheating, just a severe fundamental personality difference that meant they couldn't be married to or live with each other, there's no reason not to be friends.