I would like to have an objective, non-judgemental discussion about  the most recent episode (ep 15)  reveal that touched this topic, especially it concerns Yunnyeong, who has been well-liked participant maybe until this issue. Considering the age she had dated that guy which is ~19-20, she may be still immature relatively to the present and still navigating the constructs of dating. But at that age, I think she may have already a basic foundation of morality and ethics. Nevertheless, my opinion of her doesn't matter, but would like to read some arguments or good points if YOU were in HER position (not on JH's best friend's position).

I actually search this topic online and found more advice if you were in the best friend's POV. It said that the most considerate thing to do is to ask your best friend if it will be okay for them. It is the burden of the bestfriend to tell his/her friend about having a relationship, not the ex. (YN) It may not be the case in their situation (but who knows) as he revealed that he broke ties with that friend. 

So, going back to Yunnyeong, ofc, it would be ideal if you just eliminate the problem by not dating them, but just in case you have developed feelings for your eX's friend, do you even have the burden of informing your eX? For sure, the one with the most burden should've been JH's "best friend". But is there anything she could've done to make it have lesser impact on hurting people especially to the person who had hurt you (hence called an eX)?

For sure she could have encouraged JH's best friend she was about to date to at least inform JH about it OR to be more indubitably correct, she'll only date him provided he already talked to his friend because that would indicate something about the character of the person she is about to date. But does she need to bother herself with those things if she's not the best friend but the ex?

Riding the candy stick of your ex bf’s best friend is sick and deranged!

From her perspective she has no ties to him anymore but even if you had a falling out with your friend out of the respect you had for that friendship when it was good you wouldn't go and date their ex, and i view it the same with EX's with the exception of cheating because i wouldn't judge her if it was for revenge. but why would u even want to remain in your EX's close circle like that? it's just unecessary drama and there's so many men out there in the world. I don't think there's any way she could even lessen the impact, this would hurt him either way because he lost a friend and his EX, who he still had feelings for is with his friend while he was slaving away in the military.