I have been asked to set up a simple proof of concept to test an IOS app.
However, the app is already in the app store.
Is it possible to run tests against an app that has been downloaded to the device, from the app store?
No; it's NOT possible.
If you can do basis search here or google & you will find answers for WHY
You can find sample app for POC in github public repos
Why not instantiate driver on other app and start new activity play store. Then Download and install. Then launch and carry on test?
For Appium to connect with the iOS app, the app needs to be signed with a Development certificate. But when an app is published to the app store, it is signed with Distribution certificate. Due to this, Appium won't be able to interact with the app.