As terrible as I think Yeonju is behaving, I get where Hari is coming from. You want to be there to support your best and closest friend even when they’re acting crazy. It’s the difference between a true friend and a fair weather friend. So far, Hari is framed as the type to stick with people she’s close to even through the bad times, that’s a strength and not a weakness. She’s also shown to be selective about who she gets close to. She felt something was off about Yuha, that he was faking a smile and wasn’t truthful with her so she decided to end their dating.
Again, we’re also starting the series off when the friendship is in a bad spot, and we never got to experience the good moments. So it’s easy for us to want Hari to cut off Yeonju. That could be a drawback in writing terms Because we as the audience dislike Yeonju and don’t feel the pity and sadness Hari is probably feeling. But by starting here, we’re at the point in the story where things are most exciting/dramatic.