I don't think my reply is spoilery, but just in case it is, SPOILER ALERT!
I would sincerely hate it if they did that with his character. I'm not saying it is implausible, because it would be a twist to what people might be expecting. But I agree with Elusive, I followed that scene more as "how can you tell who that is from this far?" more than not being able to see anyone. Plus, I don't think the mechanics of that works. I can understand the girl being able to see him, but why would that guy? He had no special relationship with Soo Ha. Plus, if you look back at the very first episode, Soo Ha had the mind reading powers before Hye Sung even showed up. It is certainly an interesting theory, but one I hope is not true! Mostly because I am so hoping Soo Ha and Hye Sung get a happily ever after and if your theory is correct, that cuts down the likelihood of that happening.