One reason I think Oh Dowon is somehow involved in her drug "scandal" is that in present day the two are completely estranged. He doesn't strike me, at least from his character as a teen, as someone who would discard a friend even if she retired from acting and became a social pariah.
That they aren't seeing each other can mean several things, of course. She could have blamed him (correctly or due to some misunderstanding) and severed their ties. The two could have broken up for different reasons and not in amicable terms. She might have shunned him because she wanted to leave everything behind. But it's also a possibility that having his ascension secured, and perhaps using her as a stepping stone, he discarded her when it would be bad for his public image to associate himself with a "junkie".
It would be actually nice if he turned out to be a villain, a wolf in sheep's clothing. But I think it will be take us a long time to find out. Glancing at the list of chapters in Korean, the flashback stiil isn't over.