Soowon's dad was the bos at the furniture company and Seokjo's dad was the designer. That "your father 'cheated' on us" statement from him is because he woke up to the two said adults having a heated argument outside of his room.
He might have heard what the argument was about, or he might have not at all. But whatever the argument was about, he thinks that his dad killed himself is because of Soowon's dad fault for arguing with his father.
So what I think happened is that Soowon's dad as the bos of the company probably scammed Seokjo's dad. Or stole his patent designs and claimed as his own Or something worse.
Did I completely miss how his dad died??? What was the explanation for that?
I tried to follow this, but it was not very clear