she left hyesung when he was a child and when she got word of the news of him and dojin she came back to look for her son. they got into an argument bc she as a mother didn't look for hyesung but she also got mad at hyesung. cuz he didn't look for his mom....if I remember correctly
hyesung's dad lied to his mother that hyesung didnt wanna see her anymore, thats why she didnt look for him anymore, and thinking that hyesung didnt look for her because she hated him.
my bad i mean "because HE* hated HER*
I don't want to reread the original, but what exactly did the grandma do again I forgot.