This can go three ways:
A: She could be referring to the battle that was just taking place, which Fudou and Desumi stealthily snuck away from to progress their secret romance. In which case, the two could just wave off them sitting together as being exhausted after a long and hard fought battle (which wouldn't be a lie). She seemed pretty confident that Fudou doesn't have a girlfriend.
B: She could be referring to the fact that Fudou has a girlfriend now. Fudou does mysteriously disappear, Either way, if she does know, she wouldn't know who said girlfriend was, so Desumi may have to pass off as someone else, perhaps her high-school persona.
C: She somehow figures everything out, and after a brief explanation, shit goes down instantly.
I'm not sure why everyone's saying this manga's axed. It sounda like a climax, but it could just genuinely be a bait and switch. The first volume hasn't even been revealed yet.