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Judging by that UI screenshot it's jQuery mobile so he probably used phonegap to wrap it in a native app for both iOS and Android.

How would he use PhoneGap on a Chromebook? You still need to have X Code to sign an iOS application.

I assume his mentor was the one who packaged them up for Android and iOS (especially since it's listed under his name on the iOS App Store).

You can package the Phonegap App with Phonegap Build, a service running in the cloud for exactly this plattform-problem.