Only if they have access to a mac and xcode would it be easy. Trying to develop anything for iPhone, even a fairly simple website, can be a genuine nightmare without the proper tools for testing, let alone an actual app. I once spent over 2 days trying to solve a bug that I was able to resolve in like 5 minutes after finally admitting defeat and borrowing my brothers macbook.
Pretty sure you can get an NACS adapter now, but otherwise non Tesla vehicles pre-equiped with a NACS port are due to arrive next year.