We don't really know for sure how old the dad is. It's not a detail that's focused on in the story. But if I had to take a stab at it, I'd say he could be in his mid to late 30s.
No, it is not ethical at all. Having sexual relations with a friend's dad is a serious violation of friendship boundaries and moral norms. It can cause great harm to the friendship and also create complex family and social issues.
An old dad I knew was a big fan of baseball. He took his son to every baseball game in town. They would sit in the stands, eat hot dogs, and cheer for their favorite team. The old dad's passion for the sport rubbed off on his son, and it became a great bonding activity for them. It was a story of a dad sharing his love for a sport with his child.
Maybe it's just part of the story's character design. Sometimes, the age of a character is chosen to add a certain element or create a specific dynamic.