So... her "best-friend".

Very classic, the best-friend tries to steal your man "unintentionally". I don't like her too much for that. I mean, I get the whole uncontrollable feelings thing but GIRL that's your best-friend that you are deceiving; not to mention, you are also deceiving the guy and lies that start off a relationship will reign hell once the truth starts unraveling. My advice to her would be to just stop backing out last second, and just be honest for the better of everyone. But we all know, that won't be happening anytime soon...

What is your advice for someone who's in a similar situation as this best-friend?

Ok, this is a bit of a spoiler (!!!)... I think, for once in a kdrama (at least for me, I haven't watched it all), that the actions of this best friend are arguable. She'd like to have a taste of this thing called 'love' that she hasn't yet experienced in her own household and with other boyfriends - and since he'll leave within a few months... and the friend doesn't really seem to like the guy that much anymore anyway (or that's the impression Ha Ri gets from Hye Jin I think?). 

But of course, it's not okay to lie and deceive whatsoever. And of course there's never 'a good time' to explain the mess, but one should definitely try to be more honest with her best friend - for the sake of their relationship. So to answer your question, I'd urge this best friend to spill the beans to her best friend asap - to at least control some damage.

^ Hmm, you've got a point. But then again, I think there still would be tons of reasons for her to lie... (again, spoiler alert!!) at first Hye Jin was really annoyed with SJ's manner at the office and voiced that eloquently to Ha Ri. Ha Ri knew it was wrong to lie and meet him in secret in the first place, but she was interested in SJ - and then she fell in love which is when the reason to lie shifted...

If Ha Ri had been honest about it/herself when she and SJ met in the hotel in the first place, there would be no drama - or maybe there would be drama since SJ probably would want to know who and where his Hye Jin actually is...

Even though Hye Jin was annoyed with Seong Jun, she still had feelings for him and Ha Ri knew it.  If Ha Ri knew it was alright to date Seong Jun,  then she would have said everything openly instead of hiding everything and meeting him in secret. Ha Ri  is not innocent, but selfish. Out of all the men in Korea, she had to choose her best friend's first love. Selfish.

The things that Ha Ri has done to both Hye Jin and Seong Jun ruins friendships and relationships.  She is just trampling over all of their memories because she thinks she's in love, but she's not.  She  was so wrong, as she got stressed out and ended up in the hospital. 

Hye Jin was wrong too, because she should have never asked her friend to meet him in the first place.