Create a app (proof of concept) which uses an excisting JSON API

I want a simple and a quick creation of a proof of concept app. The app should work / integrate with an already excisting JSON API. My customer has a webapplication which tracks cars, busses, trucks, planes, boats, etc by implementing a GPS box which does updates every 60 sec. and put the location on a map. So the blackbox in the car send an update to their server every 60 sec. and that data can be retrieved by using the API. The webapp is great and should be somekind of responsive, but this doesn't work that great on a phone and thats the reason they want a native app for it. The current app developer is short in time and can't deliver and has not deliver anything yet to my or to my customer, so that is why i am looking for you. What the proof of concept should contain is a simple login page where you can enter the username and password which is created for using the API. When the information is entered correctly it should retrieve all the information about the objects and put them on the map, for an example google maps. With the API, you can retrieve the data if the engine is turned on or off and more, it is nessecary to use this data to create a green or red icon on the map if the car is driving yes or no. All this is should be very simple with the API. The API is well documented and i have a demo account to test. The Proof of concept is just a simple showcase that it can be done and then the app development will start and i will make a full case with the customer about wishes, looks, etc. For now, i need the proof of concept as soon as possible. Information about the API can be found here: [login to view URL]