Alexa, I think Jeongmin was mostly making that up to get rid of her. He didn't have an "honest" face on when he said it. But I do think he is interested in Sena's other friend. I'm confused, though--I thought the friend who is after Jeongmin is also the one who is secretly dating Seunggu's co-worker. I can't keep those girls straight.
They may be holding Jeongmin up as an example of what Seungu should have done. If Jeongmin didn't wan't someone with him at his reunion he probably could have found a way to go alone. He must think it is improper to show up at a reunion without a date but doesn't want Eunhee to get the wrong idea that he might want to go steady with her.