Responses
When you are 20 you are an adult. In most countries in the world (including Korea) you are an adult. You can choose to do sex work, drink, drive, borrow money, get married etc.
You can still be bit silly and immature but you are not a child nor a teenager in fact I've seen teenagers that know when to apologize and nobody needs to make excuses for them.

I'm sorry but how can he be so retarded?
He is afraid of losing what he has now with her but by rejecting her, he irreparably damaged their relationship. This is worse than dating and breaking up on good terms. He still has a chance to fix things but he is too much of a coward to do something.
I'm crashing out. I'd def beat the shit out of him if I was one of his bros.