They can just send Yasuho or someone else to the lecture without telling them that the head doctor will be there. Then just wait, and if it's Yasuho, she'll recognize the head doctor, and since he'll already be there no one is technically 'pursuing' him.
I dunno man, Eiji's reaction kinda disappointed me. He feels more "willfully ignorant" than "dense" from that, and I'm not sure I like it.
Then again, and I dunno if this is a longshot, but author could be setting up for some character development. He started off as oblivious and immature, then...