Can someone point me to when they established that Yoo Jung was the girl Hee Joon slept with?
@CherryBunny said it was established in the story that Bong Hee caught her now-dead ex-bf Hee Joon in the hotel lobby with the 2nd female lead, Yoo Jung.  But we haven't even met Yoo Jung yet, as far as I am aware (unless she was Ji Wook's GF in that flashback scene, I didn't really get a good look at her nor would I recognize the actress on sight anyways).

Also, it is very, very unlikely that Hee Joon was sleeping with Ji Hae before he and Bong Hee split, since she was very vocal about trashing Bong Hee for having a (fake) one-night stand.  Unless Ji Hae is a literal psychopath, she wasn't involved with Hee Joon until well after he split with Bong Hee.

Whew.  So many Korean names....
The dead boyfriend slept with Na Ji Hae. I don't think we saw her face at the lobby, but she was recognizable from her haircut and posture (dramaverse is rarely so lazy).
No Ji Wook's ex is Cha Yoo Jung. She slept with the man whom we haven't seen properly, only his back or a glimpse or him (I'm sensing a pattern here).

On the contrary, it's highly probable the deadman and Electric Fairy were already sleeping together (or at very least heading this way) and that's precisely why he was trashing Bong Hee.
I don't buy it @namopanik.  At least not yet.  It could be possible that Ji Hae was mouthing off about "kicking out anyone who has one-night stands" as a way of deflecting her own guilt for sleeping with someone else's boyfriend, but I don't really follow with that.  It just seemed like she was being super snobby just because that's who she is and didn't get involved Hee Joon until later.  I mean, it was a two year timeskip, I believe, and it doesn't fit with Hee Joon's personality to stay with one person for that long.

I could, of course, be way wrong on all of this.

STILL think it was Hee Joon that slept with Ji Wook's ex!  Leave poor Eun Hyuk alone!
The timeskip during internship? It was 2 months. If it doesn't fit with dead boyfriend personality to last 2 months, I can't imagine him still hanging around with a girl he slept with years prior (if that was Ji Wook's ex indeed).
So what Eun Hyuk did to deserve Jiwook contempt? Because it couldn't be something trifle.
Ah, well, if it was a 2 MONTH timeskip, then it makes more sense.

As for Eun Hyuk, what he did could be any number of things.  We really know so little at this point.  I have always said that "he slept with Healer-kun's ex" is the most likely scenario, but it's not one I want nor do I think it will work with what we've been given so far.  But if kdrama has taught me anything, they can make all kinds of dumb stuff work really well.
i dont think he slept with the ex, but he maybe was responsible for introducing the horner man to them.  Or maybe we are so off base it isn't funny
@Arvis:
My fault! Lol I got the name wrong. The other girlfriend, Bong Hee's rival.

She was established. We saw the back of her in the hotel. Same haircut, though we did not see her face. Then, when Bong Hee was in the police station, she started screaming at Bong Hee and even admitted ther slept together, that it was her.
When is nara gonna have dialogue tho
I have a feeling that that would be way too obvious? A lot of the viewers are suspecting he was the one who slept with Ji Wook's gf. A gut instinct tells me that that's not it, but at the same time it makes sense... 
My guess is perhaps the ex-gf found out she had a sickness and asked Eun Hyuk to stage a fake cheating scheme to 'free' Ji Wook. And maybe now she's cured and wants to get back with Ji Wook. 
@Jay

I would totally not be surprised if that were it! >_< lol
Sure. And Na Ji Hae and Jang Hee Joon were just revising for exams together.
Well this is answered in episode 10
very disappointing answer.....