I saw an ad recently for a car (Kia?) with a phone app that could sent an unlock/start code to someone else's phone. It was so you could lend your car to a friend without all the hassle of physical keys. What could possibly go wrong?
This is using NFC, which is impossible to intercept. So only the person you send the key to can unlock and start your car with their phone, and you need to hack their phone to steal the key, which I believe is impossible given the way that Apple implements it (you need to steal their phone and know their passcode to use the key unless they set it in express, and even then the phone has to be unlocked within a certain time period to actually start the car).
Assuming your friend has Internet access nothing could really go wrong. If you forget to send them the code they can just watch a short video.