Yes, it was wrong to go behind his back. But, it's not like dojin was personally involved with that sorry of a mother. And I don't think that reason was an excuse. Because I can't leave a person to fend for herself who looks exactly like my love. And that mother's most painful karma is that she can't see or talk to hyesung ever. But bringing it up in every chapter? It's nonsense tbh. But if you still want to, I can't stop u that's your personal choice.
By supporting her lifestyle with his money, he has been involved personnaly in her life to some extent... And if he saw her a someone who look like his lover -> support her, he is damn wrong. She left her child to be abused, she is complicit not a victim only. He overlook the part that make Hyesung uspet..... And why would that be karma for her ? She left him for years..... Just because she can get forgiven to her now rich son, does it mean she got real karma ? Karma is letting her starve and fend for herself... Unlike what he did.
Each their own opinion anyway ~

Can we move on from hyesung? He loves byul so so much. And dojin. He is in a happy family. And his trauma isn't brushed off. Hyesung moved on. Why do y'all want conflict in a happy family?? It's not hyesung was made to forgive his mother. He still hasn't forgive his mother (as he should). And as byul said dojin didn't even talk with her that much. This clearly means he had a strict relationship with her. Hyesung, my baby, is happy. Move on y'all.