Ok, the dude seems to be reasonable smart. We all know its Runa whos the yandere. He doesnt of course because so far theres not really any clues hinting at her being the stalker except for one. He has never gotten a threat from the stalker when around Runa, yet suddenly when around this girl he does. If this stalker was consistent, she'd also threaten to harm Runa.
If this date does happen and no threats are made, its pretty obvious shes the stalker. If he doesnt pick up on that... well thats just bad writing.