Ok so...

Honestly, I don't think Byul was wrong.
What he said may sound harsh, but it's true: abandonment is abandonment, regardless of whether it was due to fear, immaturity, or personal problems. Each person experiences that pain differently, and what Hyesung did also affected Byul.

Furthermore, I think it's unfair for Hyesung to blame him for talking to his grandmother. Just because she was a bad mother to him doesn't mean she has to be a bad grandmother to Byul. Children are not obliged to inherit their parents' traumas or grudges, nor to cut ties just because their parents decided to do so.

Byul simply held up a mirror to Hyesung: he can't ask him to reject someone “because of what they did” when he himself committed abandonment. It wasn't cruelty, it was honesty.
And the fact that he later regretted it shows that he said it out of pain, not out of malice.

In summary:
Abandonment is abandonment, and trauma is not inherited. AND I'm not saying Hyesung should forgive his mom, he cand do what it feels right, but he can't say "you can't either" that's not how life works (also both Dojin and Byul kept that hidden for valid reason 1. They knew Hyesung would be against it, because of hurt or trauma, so they didn't told him 2. They have their own criteria and judgment. They didn't brought her home like -"Hey here's grandma everything's good and forgotten" no, they kept it under wraps.

(Also that kiss of restraint... my goodness, simply beautiful, pure and cute, i love them so much(╥﹏╥)♡)