Sometimes the end justify the means. Sae-Mi had this crush for years and never took a single step, so Yena is trying to push her to act through any mean necessary.
Let me just say this : Sae-mi is lucky Yena isn't actually in love with him, because otherwise she'd have ended up seeing her crush snatched away by her. And that means that some random other girl who isn't Yena and who actually likes Young-Min could show up anytime, and actually snatch him. She's trying to teach her that lesson the hard way, because Sae-Mi is such a wuss that the soft way won't work.
If you ever had a friend who is such a desperate case that talking some sense into them won't do anything, you know how hard of a situation you're facing... and that desperate cases call for desperate measures. And in this kind of situation, there are two types of friends : the ones who are willing to take said desperate measures, and the one who would give up and let you be.
So yeah, I'd much rather take a friend who is acting like Yena rather than a friend who would say or do nothing about the situation.