Given they got suspicious about if their idea was stolen, it'd be interesting if the app solves something unique, or some very common problem.
Yeah I'm fully expecting it to be something like a project management app. There are tons of ideas which you think are unique, but many, many others have them as well, because they run into all the same problems as you in their day to day life.
I remember he at some point had a landing page for a browser that would show you a web page in multiple resolutions, all at once, with hot reload. Not sure if this is the application, he might have had another one.