Yi-Hweok's mum is a lesbian and had an arranged marriage with his father. He raped her and she gave birth to Yi-Hyeok with a lot of resentment and was extremely suicidal with post-partum depression. She tried to maintain that her marriage was normal as a coping mechanism. She met Inseo's mum at a post-partum centre and they became quick friends. Inseo's mother is probably the reason why she's still alive tbh. 

Yi-Hweok's mum fell in love with her, but did not realize those feelings until much later when Inseo pointed this out to her one day. She had a breakdown when she realized that because Inseo knows, then it's likely his mum knows too. She just wants her friend to be happy and doesn't want to ruin anything, essentially deciding that if she leaves and is out of the picture, then everything will go back to normal. 

She regrets leaving Hi-Yweok. She clearly loves him very much and doesn't expect him to forgive her. 

ALSO THE BOSS HIT HI-YWEOK'S MUM WITH HIS CAR ON PURPOSE. HE'S FUC**** CRAZY.