If you are an Apple only smartphone (or tablet) user then you likely buy your movie content straight from the king himself but there may be times when the content is only available on Google Play Movies or that it is simply cheaper there, Would you be surprised to learn that you can play Google Play Movies on IOS with the original Google Youtube app?
First caveat is that you can only stream content using this mechanism (aka you won't be able to download the content and play it while offline).
Step by Step
Log-in to the Youtube app using your Google account
Click on the hamburger icon
Then Click on the little arrow next to your name
Then click on Purchases
You are the presented with all the Google Play Movies you have purchased and can immediately stream them at any resolution (up to the max resolution you purchased SD or HD).,
This is one more way to acquire and consume content on your IOS device.