How to Disable FLAG_SECURE without root?
You might want to give Virtual Xposed a spin. 

It is an application that acts as a sort of "host" for other user apps you can install inside it. It then has full control over the process of the inner app, and can apply the functionality of some Xposed apps without requiring an unlocked bootloader or Root. 

However, it can only work for Xposed modules that don't modify the system. If your mentioned module tries to do that, VX won't be any good.