How do I install .IPA files on my Iphone 7?

So one of my friends is trying to develop his own app and he wants my help to test it.

He gave me a copy of the app to install onto my Iphone so I could try it myself, but as far as I can tell it is impossible to install it without special tools.

My friend was able to install it on his phone using the program he used to build the app. But since he lives over 4 hours away popping over to his house to have him install it isn't really an option.

Is there no way to install apps that are not currently in the Itunes store?